Well how many Doctors we gets depends on how long BBC wants to keep doing the show. For now it's got enough of a fan following, will that change? Or will the Doctor after Matt Smith leaves be the one that breaks the camel's back? Or will it be time to 'reboot'? I still find it amazing that Dr Who has been on the air semi-continously since even before Star Trek so that's astonishing in and of itself. I mean 50 years is unheard of for anything SF/F. (Yes I know there was that gap but even so there was still 'official' Who media out, just not an active show. Plus maybe it was a good thing it went on a sort of indefinite "hiatus", nuWho benefited from more modern film making techniques, special effects and hey it gave a Star Trek-less world another bit of sci-fi media to latch onto again.
I'm just curious though, has it been brought up in the current modern era that he's only got 13 regenerations? I mean, did Ecceleston or Tennent or Smith ever say that he's got 13 lives? Now that's technically the last of the old Time Lords, maybe some of the old rules no longer apply. Tennant sorta proved that when he declared himself Time Lord Victorious. So... who knows.
Although I do feel like, especially with Smith's Doctor, that he's starting to 'feel his age' a bit. Granted a good mystery can break him from his funk, but even so... to paraphrase Yoda when over 900 years old you reach, feel as good you will not hmm?