According to this source (which isn't the most reliable of newspapers but they seem to say that they spoke to Steven Moffat) it'll be revealed in the Christmas special that Matt Smith is the 13th Doctor - Hurt's Doctor bumps him from 11th to 12th and the extra regeneration to save himself in Journey's End (the one where Rose returns and the Doctor gets hit by a Dalek causing the regeneration which is then channelled into his 'spare hand' creating another Doctor) bumps him to 13th. The Christmas special will deal with the regeneration issue (that Time Lords only have 12 regenerations) and that the Doctor has used them all up.
Of course we know the Doctor will carry on as Peter Capaldi. We also know that Time Lords can survive beyond 12 regenerations - the Master used all of his up (The Deadly Assassin back in the 70's) and then started stealing bodies to continue (The Keeper of Traken, The Doctor Who Movie) and was granted more regenerations by the Time Lords if he fought for them in the Time War (as mentioned in Utopia/The Last of the Time Lords). We also know that River Song used up the rest of her regenerations to save a dying Doctor (Let's Kill Hitler).
So there are plenty of options for keeping the Doctor going for another 50 years.