Smith is the 10th Regeneration, 11th Doctor. The doctor has 13 lives, 12 Regenerations. Though, if ratings are still favorable, the Showrunner will either ignore this piece of canon or write him more lives (Sarahjane Adventures already had a throwaway line that he had over 500 lives, which may or may not have been serious)
And just to throw even more baggage onto the whole 12 limit regeneration, the time lords it seems can grant another 12 full regeration which they offered him in the 25th anniversay The five doctors.
Plus add to that the masters ability over the years to be able to steal people bodies, lifes and regenerations.
Best just to accept it all and just enjoy it.
He said, much as Andrew Cartmel would years later, that he wanted to add some more mystery to the character of the Doctor.
Plus if she did have any regens left would she not have done so when she died in Forest of the dead, or even worse did Tennets doctor unintentionally trap a fellow timelord in the virtual world thing before she had time to regen.
