In the latest issue of the Radio Times, Steven Moffat has revealed that Matt Smith’s Eleventh Doctor is actually the thirteenth incarnation (counting the War Doctor and the Tenth Doctor’s Meta Crisis back in The Stolen Earth – and by the rules set down back in The Deadly Assassin, the Doctor can only regenerate thirteen times.
“We’ll find out that Matt Smith is actually the 13th Doctor. Although everyone knows that the Doctor can only regenerate 12 times. The 12 regenerations limit is a central part of Doctor Who mythology – science fiction is all about rules, you can’t just casually break them. So if the Doctor can never change again, what’s Peter Capaldi doing in the Christmas Special?”