Doctor Who: A Regeneration FAQ
The limit on regenerations seems to be a traditional one, not necessarily a biological or physical one. It is said that the limit exists to prevent bio-decay, basically loss of connection with the original form. It is important to note that showrunner Steven Moffat recently acknowledged that the limit does exist.
If we consider that we already know Peter Capaldi is the Twelfth Doctor, and that John Hurt is also an incarnation of The Doctor, then that means Peter Capaldi should possibly be the last if convention is held to. However, fellow Time Lord The Master has had as many as 17 possible incarnations, and the High Council of Time Lords appears to have the ability to grant more if it so desires. The ability of River Song to revive The Doctor by expending all her regeneration energy, as well as him using his own to heal her broken wrist, implies that there is not so much a finite number as a finite amount of energy. Perhaps that energy can be recharged. I suspect we'll find out very soon.
I read this novel called Lungbarrow and it said...: For more information on how stories and mythologies don't line up and may in fact be wildly different from other stories, please see the history of all Earth's religions.
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