I remember back in the Tom Baker days, the rule was that Time Lords were limited to twelve regenerations (there was an entire storyline about the Master using up his 12 and trying to earn more from the Time Lords), but I believe this was a limitation imposed by the Time Lords themselves rather than a biological limitation. Since it hasn't been mentioned in the new series, I think we are to assume with the destruction of Galifrey (or it being stuck in the time loop or whatever) that the Doctor's regenerations are unlimited since there are no other Time Lords to enforce the twelve rule. (Show's what a geek I am that I actually know this...)