The Master's regeneration cycles

The Master had several regeneration cycles making up their many lives, as well as some regenerations achieved through other means.

Original regeneration cycle
Their original life cycle, lasting twelve regenerations following the Master's first incarnation, was used up at a faster rate than most Time Lords. (TV: The Deadly Assassin)

Accelerated use
The Master used their original regeneration cycle at an accelerated rate compared to other Time Lords. (TV: The Deadly Assassin) Various accounts offered explanations for this.

The Fourth Doctor believed that the accelerated use came from the Master living a high pressure criminal life, with regenerations used as disguises. (PROSE: Doctor Who and the Deadly Assassin)

Magnus was notably frivolous with his regenerations. (COMIC: Flashback, PROSE: Invasion of the Cat-People)

claimed that he'd wasted all of his lives because of the Doctor. (TV: Doctor Who) An account of this occurred in Koschei's meeting with the Second Doctor, which ended in Koschei expending several lives to escape an event horizon. (PROSE: The Dark Path)

A Brief History of Time Lords believed that the accelerated use of regenerations came from living a life "more rackety than the Doctor's". (PROSE: A Brief History of Time Lords)

First incarnation
By one account, Koschei was the Master's first incarnation, who received the regeneration cycle when he got the Rassilon Imprimatur after graduating from Prydonian Academy. (PROSE: Divided Loyalties)

In an alternate timeline, a young Master who had yet to leave Gallifrey was one of several incarnations who gathered on Kiamet. (AUDIO: Masterful) By one account, Missy remembered that the Master had left Gallifrey by the time of their first regeneration. (PROSE: Dismemberment)

CIA File No. 197,648,200/7A indicated that the Master's first incarnation lived on Gallifrey before the Master became a Renegade Time Lord. After attending the Prydonian Academy, the Master began conducting field work studies of historical interplay and temporal structure through use of intervention, during which he regenerated. (PROSE: CIA File Extracts)

Second incarnation
After the Master regenerated for the first time, by Missy's memory, his new incarnation found himself at the Scoundrels Club during the Great Fire of London. Becoming a member of the club so that he could recover from the regeneration in comfort, the Master organised fireworks on the roof to celebrate the occasion. He would later go on to visit the Scoundrels Club after each regeneration to recover as a tradition. (PROSE: Dismemberment)

CIA File No. 197,648,200/7A indicated that the Master's second incarnation lived on Gallifrey before the Master became a Renegade Time Lord, regenerating during his time conducting field work studies of historical interplay and temporal structure through use of intervention. (PROSE: CIA File Extracts)

Third incarnation
CIA File No. 197,648,200/7A indicated that the Master's third incarnation lived on Gallifrey before the Master became a Renegade Time Lord, regenerating during his time conducting field work studies of historical interplay and temporal structure through use of intervention. (PROSE: CIA File Extracts)

The Fourth Doctor was unsure if an incarnation of the Master he met was on his second or third "regeneration". (AUDIO: Blood of the Time Lords)

Fourth incarnation
CIA File No. 197,648,200/7A indicated that the Master's fourth incarnation lived on Gallifrey before the Master became a Renegade Time Lord, regenerating during his time conducting field work studies of historical interplay and temporal structure through use of intervention. (PROSE: CIA File Extracts)

The Fourth Doctor was unsure if an incarnation of the Master he met was on his second or third "regeneration". (AUDIO: Blood of the Time Lords)

Fifth incarnation
CIA File No. 197,648,200/7A indicated that the Fifth Master was responsible for the Prydonian Academy Revolution, and was mortally wounded by Arkendo when he fled Gallifrey to become a renegade. (PROSE: CIA File Extracts)

Sixth incarnation
CIA File No. 197,648,200/7A indicated that the Master's sixth incarnation was a renegade known as the Monk. After meeting the First Doctor in 1066, this incarnation was killed by being trapped in a shrinking TARDIS. (PROSE: CIA File Extracts)

Seventh incarnation
CIA File No. 197,648,200/7A indicated the Master's sixth incarnation regenerated into an identical-looking seventh incarnation also known as the Monk, who faced the First Doctor a second time. This incarnation discovered Merast, the Master's secret base. This incarnation was killed by the Daleks following his role in the Time Destructor Incident. (PROSE: CIA File Extracts)

Eighth incarnation
CIA File No. 197,648,200/7A indicated that the Master's eighth incarnation was the first to call himself "the Master". Once more with a bearded face like that in which he would face the Third Doctor, the Master began enacting various plans of universal domination, some of which succeeded and some of which ended in failure, sometimes due to Celestial Intervention Agency intervention. He suffered a setback which resulted in his regeneration. (PROSE: CIA File Extracts)

Ninth incarnation
CIA File No. 197,648,200/7A indicated that the Master's ninth incarnation, who resembled the thirteenth Master, enacted various plans of universal domination, some of which succeeded and some of which ended in failure, sometimes due to Celestial Intervention Agency intervention. He suffered a setback which resulted in his regeneration. (PROSE: CIA File Extracts)

Tenth incarnation
CIA File No. 197,648,200/7A indicated that the Master's tenth incarnation, who resembled the thirteenth Master, enacted various plans of universal domination, some of which succeeded and some of which ended in failure, sometimes due to Celestial Intervention Agency intervention. He suffered a setback which resulted in his regeneration. (PROSE: CIA File Extracts)

Eleventh incarnation
CIA File No. 197,648,200/7A indicated that the Master's eleventh incarnation, who resembled the thirteenth Master, enacted various plans of universal domination, some of which succeeded and some of which ended in failure, sometimes due to Celestial Intervention Agency intervention. He suffered a setback which resulted in his regeneration. (PROSE: CIA File Extracts)

Twelfth incarnation
CIA File No. 197,648,200/7A indicated that the Master's twelfth incarnation, who resembled the thirteenth Master, enacted various plans of universal domination, some of which succeeded and some of which ended in failure, sometimes due to Celestial Intervention Agency intervention. He suffered a setback which resulted in his regeneration. (PROSE: CIA File Extracts)

According to some accounts, this incarnation or an even earlier incarnation may have been the incarnation that faced the Third Doctor. (PROSE: Legacy of the Daleks, COMIC: Doorway to Hell)

Thirteenth incarnation
According to some accounts, this incarnation was the incarnation that faced the Third Doctor; according to the CIA File No. 197,648,200/7A, with many plans unfolding from the activities of his past few incarnations, including several connections formed for future alliances, it was this final Master who set his sights on Earth.. After being wounded during the Second Dalek War, the Master's handsome features began to decay, leading to this same thirteenth incarnation surviving as the Decayed Master. (PROSE: CIA File Extracts) By another account, however, the Master was instead burned severely by Susan Foreman; this account did not clarify this incarnation's regenerative status one way or the other, merely claiming that "there was no way to regenerate from such a death". (PROSE: Legacy of the Daleks) Yet another account specifically depicted the "UNIT era" Master as capable of regeneration, with him beginning to glow with a golden light after a battle with the Twelfth Doctor. (COMIC: Doorway to Hell)

When facing the Fourth Doctor on Traken, the Decayed Master claimed that he was "nearing the end of [his] twelfth regeneration". (TV: The Keeper of Traken) By some accounts, this Master was created when the Master attempted a thirteenth regeneration beyond their allotted twelve. (PROSE: Divided Loyalties, Meet Missy!, The Doctor vs the Master)

The Decayed Master understood that he needed a vast energy source to fuel a new regenerative cycle. (PROSE: Cold Fusion) On Gallifrey, the Master attempted another regeneration, (TV: The Deadly Assassin) but only succeeded in partially healing himself with energy from the Eye of Harmony. (AUDIO: Trail of the White Worm) Later attempts involved a plot to rend asunder the constellation of Mandusus using a segment of the Key to Time, and entering a pact with the Embodiment of Gris; both were thwarted by the Doctor. (PROSE: Cold Fusion)

Post-mortem & legacy
When the "Tremas" Master was stripped of his Trakenite body by the Time Lords, and after his plot to steal the Fifth Doctor's regenerations failed, he found himself confronting mental projections of all his past incarnations, and was able to steal a bit of life energy from each of them, allowing him to regenerate back into his Trakenite body. (PROSE: The Velvet Dark)

The Master would spend considerable time attempting to gain a new regeneration cycle to continue surviving, sometimes mentioning their original regenerative cycle. (PROSE: Cold Fusion, The Quantum Archangel, et al.)

Tzun-granted life cycle
The Tzun gave the Tremas Master a new regeneration cycle, which resulted in a regneration which created a new Gallifreyan Master. (PROSE: First Frontier)

From body to body
The Master acquired the physiology of a Deathworm Morphant to survive his execution by the Daleks. (AUDIO: Mastermind, TV: Doctor Who) After using this form to possess a human body, Bruce Gerhardt, understood that this body was beginning to decay, and so tried to use the Eye of Harmony to steal the Eighth Doctor's remaining regenerations to heal himself. His plans were foiled when Grace Holloway sent the TARDIS into a temporal orbit. The Master ended up falling into the Eye of Harmony, (TV: Doctor Who) but eventually escaped the TARDIS, finding that his human body had been "energised" by the Eye. (AUDIO: Day of the Master) Ultimately, this body was destroyed by the Time Vortex. (AUDIO: Passion)

Nearing his ultimate destruction, the Master was rescued from the Vortex by a being named Esterath. After gliding over the many realities throughout the Omniversal Spectrum for what he described as seeming like "centuries", the Master was resurrected into the body of a recently deceased vagrant on the streets of 2001 Brixton. Some weeks afterwards, due to a symbiotic link he had formed with the Doctor's TARDIS when it consumed part of his essence after he passed through the Eye of Harmony, the Master was transported onto the Moon during one of the Doctor's adventures. The Master subsequently used this link to trail the Doctor for some time without his enemy suspecting. The Doctor's companion Kroton, after becoming the controller of the Glory, cleansed the TARDIS of the Master's influence and placed the Master somewhere that he could not escape. The Master declared he would survive and return. (COMIC: The Glorious Dead)

Once more trapped within the Eye of Harmony, the Master eventually escaped by influencing the dreams of Edward Grainger to unravel the Doctor's timeline, by killing Edward Grainger whilst he was an infant in 1906. (PROSE: Forgotten) Though his Morphant form had long since sublimated to a gaseous state, the Master was still capable of using its powers to possess new bodies. (AUDIO: Mastermind) However, while in the body of Sir George Steer, he was stopped by an older Edward Grainger from 2006 and Violet after being hit with a rolling pin and being removed from George. (PROSE: Prologue to The Centenarian) The Master then managed to evade the Doctor's detection, and possessed the body of a human named Richard. (PROSE: Forgotten)

However, the Master discovered his possession had caused the host body to decay at an accelerated rate, so he was forced to steal more bodies to prolong his survival. Arriving in New York City, the Master took possession of a member of the Hudson Dusters, quickly becoming the leader of the gang and calling himself "Don Maestro". After twenty years of living in his current body, he occupied the body of his host's son, Michael, and moved to Las Vegas where he owned a casino. He accumulated money to fund experiments towards the elongation of the lifespan of his host body. Fearing the eventual decay of his body, the Master used his money to buy a penthouse to isolate himself from infection. After years living in isolation, his host's son confronted him with the knowledge that he had possessed both his father and his grandfather in some way. He then trapped the Master in the penthouse.

After UNIT were alerted to the presence of penthouse, they discovered the Master in a comatose state. He was imprisoned in the UNIT Vault, awakening every five years for one hour, before returning to a coma. After fifteen years living in the Vault, the Master awoke for a third time and was interrogated by UNIT officers Ruth Matheson and Charlie Sato. However, he managed to hypnotise both of them and escape his imprisonment. Discovering that UNIT had recovered his TARDIS from a sealed tomb in the Valley of the Kings, he used it to escape from the Vault. (AUDIO: Mastermind)

Resurrected regeneration cycle
By some accounts, the Master's resurrection led to them being granted a new regeneration cycle, with the Decayed Master regenerating into the Reborn Master. This was done with the assistance of two future incarnations, the War Master and Missy, as well as the earlier Master in the body of Bruce. (AUDIO: Day of the Master)

In one account of the end of the Last Great Time War, the TARDIS had long "suffer[ed]" since the High Council's resurrection of the Master before it eventually took a new shape following the destruction of Gallifrey Original, which led to the Eighth Doctor's regeneration. (PROSE: Doctor Who and the Time War)

The War Master, after escaping the Last Great Time War, was forced to regenerate into the Saxon Master after being shot by Chantho. (TV: Utopia)

Following the Year That Never Was, the Saxon Master was himself shot by Lucy Saxon but refused to regenerate, choosing to die rather than live with the Tenth Doctor, who proceeded to burn the Master's body. (TV: Last of the Time Lords) However, the Master had made arrangements for the Disciples of Saxon to resurrect him using salvaged biometrical signatures. This process was interrupted by Lucy Saxon, resulting in the resurrected Saxon Master burning up his own life force. He was embroiled in the Ultimate Sanction, which ended with him returning to Gallifrey at the end of the Time War. (TV: The End of Time)

As the Twelfth Doctor learnt, the Time Lords had cured the Master's "little condition" before "kicking" him off Gallifrey. The Master ended up trapped on a Mondasian colony ship where he found his future self, Missy, with the Doctor. Refusing to stand with the Doctor, the Master used his laser screwdriver to shoot Missy on full blast, preventing her from regenerating, but not before she inflicted a mortal wound by stabbing him, forcing him to regenerate into her. (TV: The Doctor Falls)

Elysian field regeneration cycle
According to one account, Missy used an Elysian field to obtain a new regeneration cycle to survive her death. This resulted in the creation of a new incarnation, the Lumiat, who sought to do good. However, the Lumiat was shot by a younger version of Missy after attempting to interfere in the schemes of her younger self. Forced to regenerate, the dying Lumiat was dumped by Missy on a random planet. (AUDIO: The Lumiat)

Following Missy, (PROSE: The Wonderful Doctor of Oz) the Spy Master repeatedly menaced the Thirteenth Doctor and ultimately enacted a plot to take over her body and identity by subjecting her to a forced regeneration. This was initially successful, with the Doctor's body changing to reflect the Master's form, but was undone by Yasmin Khan using regeneration energy taken from the CyberMasters. This forced the Master back to his own body, which began to fail as a result of his actions. He was left for dead, crushed by a boulder next to his his TARDIS. (TV: The Power of the Doctor)

Possible futures
Whilst exposed, the heart of the Master's TARDIS showed him some of his possible futures. In one the Master was horribly deformed, being cared for in a Zero Room on Gallifrey after being rescued by Chancellor Goth. In another, however, the Master achieved his aim of conquest, but now possessed an entirely alien body. (AUDIO: The Threshold)

A "listless-looking" Ninth Doctor who existed as a separate future for the Eighth Doctor from the "man with big ears" (PROSE: The Tomorrow Windows) was the contemporary of a male incarnation of the Master with a black beard and wild hair, who wore an outfit with a long cloak and a large green collar. (TV: The Curse of Fatal Death)

Alternatively, an pale, aristocratic Ninth Doctor (PROSE: The Tomorrow Windows) was accompanied in the TARDIS by a bearded Master who now resided in an android body. (WC: Scream of the Shalka)

In a timeline in which the Saxon Master sought to avert his regeneration, the gathered incarnations of the Master were faced with an entropy wave that threatened to destroy and consume the universe. However, the War Master eventually deduced the wave was actually their final form. This timeline was aborted when Missy, after the other Masters had been killed, was consumed by the entropy wave, thus creating a paradox. (AUDIO: Masterful)

Parallel universes
In the Unbound Universe, the Master who faced the Third Doctor regenerated into a different incarnation. (AUDIO: Sympathy for the Devil'')

Behind the scenes
The Doctor Who Role Playing Game: The Master claimed that exposure to the Numismaton Gas of the planet Sarn as seen in Planet of Fire gave the Tremas Master considerable powers which included the ability to re-trigger a life cycle of regenerations in his non-Gallifreyan body. Escaping his fiery death, the Master regenerated into a fifteenth incarnation with a form identical to that of his previous incarnation.