Doctor Who showrunner Steven Moffat has been talking to Radio Times about the future of the Doctor, given the character is now dangerously near the end of his regeneration cycle as spelled out in the 1976 story The Deadly Assassin.
The story stated a Time Lord can only regenerate twelve times, and Moffat considers the Doctor is now in his thirteenth and final body. John Hurt is now officially a Doctor, sitting between the Eighth and Ninth Doctors, and David Tennant used up an extra regeneration during his stay. The writer was clear about the future, and how the Doctor is now at the end of his cycle - something that could cause problems when Matt Smith turns into Peter Capaldi in the 2013 Christmas episode. "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.."
To add to the confusion and despite the number of bodies the Doctor has had, Moffat is clear that Matt Smith is still the Eleventh Doctor. He told a press conference at London's Excel arena that the addition of the John Hurt Doctor to the series does nothing to alter the numbering of the other incarnations.
"He's just The Doctor, Matt Smith's Doctor is the 11th Doctor, however there is no such character as the 11th Doctor – he’s just the Doctor – that's what he calls himself. The numbering doesn't matter, except for those lists that you and I have been making for many years. So I've given you the option of not counting John Hurt numerically - he's the War Doctor."
Moffat also talked of how he found it irresistible to put Tom Baker into the 50th Anniversary story. "Tom didn't want to come and do a long thing and he didn't want to put the old costume on. He didn't want to do any of that, but his agent said that he wasn't against the idea of doing a short appearance."
Baker was the longest-serving Doctor, appearing from 1974-1981. Moffat said it would have been impossible to include all the surviving Doctors in the episode. "You can't have scenes around 11 or 12 people - you can't do it. To have the longest-standing Doctor make an appearance and be the one who briefs the new Doctor on where to go . . . well, it's irresistible, isn't it? You get to hear that voice again! It was just wonderful."