Regeneration is an integral part of the lore of Doctor Who, but how many times can a Time Lord actually regenerate? First introduced during season 4 of the classic series, the Doctor's regeneration was not only a clever piece of sci-fi, but it also allowed the show to replace its aging star. Since then, the Doctor has gone through many different regeneration scenarios, and each version of the Time Lord has had their own unique personality and quirks. Doctor Who has established that all Time Lords can regenerate, but the Doctor's unique journey through his phases is the heart of the series.
During the events of the Doctor Who serial "The Tenth Planet," the First Doctor was overcome by his advanced age and quickly transformed into the much younger Second Doctor. Though every Doctor would react to their regeneration differently, the process is repeated every few seasons across both the classic and revival series. What started as a mere convenience has become a story device all its own, and the Doctor's regeneration cycle usually results in some of the best episodes of the series. Despite all that, the cycle has been shown to be finite, and even a Time Lord has their limits of regeneration.
A Time Lord Can Regenerate 12 Times
Though the classic Doctor Who series only got up to the Seventh Doctor before it was canceled, the show had already established a cap on how many times the Time Lord could regenerate. In the season 14 serial "The Deadly Assassin," the Fourth Doctor met up with his fellow Gallifreyans, and it was divulged that he and all of his species could only regenerate 12 times. Though Time Lords were a clumsy retcon when they first debuted at the end of season 6's "War Games" serial, they nevertheless helped create a backstory for the Doctor and expanded the lore of the franchise.
With a hard limit set on regenerations at 12, that meant that a Time Lord could inhabit 13 different incarnations before eventually suffering permanent death. That fact was then restated in the season 20 serial "Mawdryn Undead," which finally established it as part of the canon and made ignoring it much more difficult for future writers. At the time, the Doctor was only up to his Fifth incarnation, and the possibility of ever reaching a Thirteenth Doctor seemed totally out of the question. Nevertheless, the revival series picked things back up with the Ninth Doctor, and each regeneration brought him closer and closer to his final rebirth.
The Doctor Earned Another Regeneration Cycle
During one of Doctor Who's best special episodes, 2013's The Time of the Doctor, the Eleventh Doctor was shown extremely advanced in age after years of fighting the Daleks on Trenzalore. Though he should have had two additional lives, he explained that The War Doctor and the Tenth Doctor's aborted regeneration meant that he was technically the Thirteenth after all. At the behest of Clara, the Doctor was granted another cycle that would have presumably meant 13 more. However, Doctor Who season 12, episode 10, "The Timeless Children," revealed that the Doctor actually predated Time Lords and has an unknown number of regenerations.