The Master's regeneration cycles

The Master's original regeneration 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 his 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) 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)

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)

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)

Thirteenth incarnation
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, the Master's thirteenth and final incarnation set his sights on Earth. This was the body in which he faced the Third Doctor. After being wounded during the Second Dalek War, the Master's handsome features began to decay, leading to the thirteenth incarnation surviving as the Decayed Master. (PROSE: CIA File Extracts)

The Decayed Master once claimed that he was "nearing the end of [his] twelfth regeneration". (TV: The Keeper of Traken)

Attempted fourteenth incarnation
By some accounts, the Decayed Master was created when the Master attempted a thirteenth regeneration beyond their allotted twelve. (PROSE: Divided Loyalties, Meet Missy!, The Doctor vs the Master)

Incarnations of ambiguous numbering
Several accounts indicated the incarnation of the Master who faced the Third Doctor was capable of regeneration, indicating him to not be the thirteenth incarnation. (PROSE: Legacy of the Daleks, AUDIO: Sympathy for the Devil, COMIC: Doorway to Hell)

Post-mortem
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)

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