As I understand it, the Dr gets 12 "lives" and we're currently on the 11th. What happens at/after the end of the 12th?
I would guess this, like Trek issues, is a Who-geek arguing point? Or is it known? Does he just die at that point? What if the show is successful when whoever the 12th Doctor is decides to leave and do something else?
Actually, the established rule is 12 Regenerations, so 13 lives.
In the Classic Series, there is precedent for getting around that:
1. The Master took over someone else's body
2. The Master was offered a full set of new Regenerations by the Time Lords
3. The Audios and/or books, I think, have a couple more times when Master escapes death
4. At least twice in the Classic Series Villians have tried stealing his remaining Regenerations (Of course, the Doctor wouldn't steal anyone's lives, but, some accident or event could somehow transfer them to him)
Also, there is a throwaway line in the Sarah Jane Adventures where the Doctor says he has 519 lives or something ludicrous, so, that could just be him joking around, future Writers/showrunners may ignore that line, or they can always say when he ended the Time War, somehow he absorbed all their future Regenerations, some kind of freaky Radiation event could be used to Retcon him new Lives, endless possibilities.
If they want to continue Doctor Who when the 13th Actor is used up, there is any number of possibilities they could use to let him continue on.