The Doctor's new regeneration cycle

The Doctor was granted a new regeneration cycle, (TV: The Time of the Doctor) also called a new life cycle, (PROSE: Regeneration) at the end of the Eleventh Doctor's life, allowing them to live beyond the First Doctor's regeneration cycle. (TV: The Time of the Doctor)

Origins
The High Council only gained the ability to grant new regeneration cycles from the research of Ophiuchus. The Doctor would attribute the existence of their new regeneration cycle to the Fifth Doctor's encounter with Ophiuchus. (COMIC: Ophiuchus)

The Eleventh Doctor was originally to be the Doctor's final incarnation. On the verge of dying of old age during the siege of Trenzalore, the Doctor's companion Clara Oswald pleaded with the Time Lords through a crack in the universe to change the future and save the Doctor. In response, the Time Lords gave the Doctor a new life cycle by sending him regeneration energy through the crack. (TV: The Time of the Doctor)

The Cycle's extent
When Captain Lundvik threatened to shoot him, the Twelfth Doctor speculated that he would "keep on regenerating forever" if he was executed. (TV: Kill the Moon) Rassilon would later state his own uncertainty to the number of regenerations the Doctor had been granted, (TV: Hell Bent) with the Saxon Master and Missy also considering their own uncertainty when debating whether to throw the Twelfth Doctor off a roof, believing they "could [be] up and down the stairs all night." (TV: The Doctor Falls)

Used regenerations
Receiving the new regeneration cycle caused an explosive "reset" in the Eleventh Doctor's body which restored the him to his youthful form and destroyed the attacking Dalek forces. From here, the Eleventh Doctor's regeneration occurred shortly afterwards. (TV: The Time of the Doctor) Eventually, this too was followed by the Twelfth Doctor's regeneration into the Thirteenth Doctor (TV: Twice Upon a Time) and the Thirteenth Doctor's regeneration into the Fourteenth Doctor. (TV: The Power of the Doctor)

Behind the scenes
Taking cue from The Time of the Doctor's wording, various Twelfth Doctor material released in 2014 refer to this regeneration cycle as “a new regeneration cycle”, including the instructions to LEGO 21304 Doctor Who, the author’s notes for The Crawling Terror, Doctor Who The Official Annual 2015, and various issues of Doctor Who Magazine.

Though unconfirmed, it is very likely that the Timeless Child revelation has rendered the new regeneration cycle moot.