Matt Smith will be the 11th Doctor - a term that only exists outside the series's 'reality' (I'm pretty sure that while the Doctor's occasionally said he's in his nth incarnation, or has regenerated n times, he's never said 'Hello, I'm the nth Doctor'). Phrases like Nth Doctor exist only in PR, articles and reference books.
But the Doctor has now used 10 of his 12 regenerations (with the 10th being half-used to heal the Tennant Doctor, and half saved down to the severed hand, only to be transferred into Donna and then to the Doctor duplicate), so he only has one regeneration and hence two incarnations to go (ie, Matt Smith and one more).
BUT... explaining in every PR release how Tennant actually played two incarnations is not going to happen, and you can get round SE/JE by saying that a regeneration is only 'spent' if it's completed (even if the whole internal logic of the sequence says that regeneration is gone - it started, he vented it into the Hand, the Hand then created an entire duplicate Doctor out of the saved energy!). How it's actually dealt with in the long term is a bit of small print to be handled by whoever's in charge at the time it all gets past the 12 regen limit, whether that happens in two or three Doctors' time.
And we can then retcon around it if we're bothered.