He travels through time and space, saves the Earth, and has millions of fans all over the world. But as every "Whovian" knows, the Doctor cannot last for ever: Time Lords are able to regenerate only 12 times before they die.
Fans have always thought that the 13th doctor would be the last, thanks to a 1976 'Doctor Who' episode, "The Deadly Assassin", featuring Tom Baker as the Doctor in his fourth incarnation, and revealing for the first time the regeneration limit. But a passing comment in a children's television programme [rewrote] history and cast the Doctor, iconic hero of the world's most successful and longest-running science fiction series, as immortal.
But in the spin-of series 'The Sarah Jane Adventures', the situation may have been addressed when Clyde pestered the Doctor about his ability to regenerate:
One thing to remember, as River Song pointed out - The Doctor lies. He's lying about how old he is, for one - he's much more than 905 years old. And I would figure that the Doctor is lying to Clyde as well. But only to shut him up with his pestering questions.
The thing is, I think the Doctor knows that the original limit for the number of regenerations for a Time Lord has been expanded in his case, but he just doesn't know by how many. (And I think that would be preferable. It would be like a human knowing exactly when they would die - would you really want to know?)
And I think the Doctor's life-span has been expanded because he absorbed the energy of the Time Stream, as seen in the episode "Parting Of The Ways":
Hopefully, the topic can now be dropped. Why bother bringing up exactly how many times the Doctor can regenerate? It's not like we're ever going to find out for ourselves what happens after the role has been cast for the 507th regeneration. Look how long it took us to get to Number 11!