Doctor Who is a Timelord, and as it’s been stated many times over the years TimeLords can regenerate 12 times. It is a fixed number! Now counting the original Form + 12 Regenerations A time lord basically has 13 Lives. After that he is no more, Unless like the Master he steals other people bodies, or other such things to keep living. This is set in stone Doctor Who cannon “Twelve Regeneration”

Some people were starting to worry that with Matt Smith stepping down we were running out of the number of regenerations left to the Doctor, which would then lead to major problems when future actors step down.

Then to make matters more interesting Moffat Slipped in a New Regeneration. We never saw Paul McGann (8th) regenerate into Christopher Eccleston (9th).  Now we find out that there was a regeneration between them “The War Doctor” the incarnation the Doctor hides even from himself. So from 9 upwards we now have to add 1. Mr Smith goes from 11th Doctor to 12th. We are still ok for the Xmas regeneration since there can be 13!

BUT. Tennant used up a regeneration to save off death after being shot by a Dalek. Which makes Mr Smith the 13th and final Doctor. (A lot of people were saying the Tennant extra regeneration did not count, since he did not change form. Moffat has now admitted that it does count and Mr Smith is the 13th)

Doctors:

So how are they going to fix it?

First off lets remind ourselves why there is a limit to the number of times a Timelord can regenerate. It has been stated that a Timelord has a fixed amount of regeneration energy inside them, enough for 12 regenerations. Although it can be used for other stuff, like The Doctor using a small part of it to repair rivers broken hand in the Daleks take Manhattan.

1: Sisterhood of Karn

The writers could have claimed the “War Doctor” did not use up a regeneration since the Sisterhood of Karn held off his death and gave him a choice of what he wanted to be. And they are the keepers of the Flame of Eternity. This would only give them one extra regeneration, so not a good long term solution. +1 Regerneration.

3: Bad Wolf

Another option they could pick (And we know how they like going back to Rose/Bad Wolf). When Rose took the Time Vortex inside herself and became the Bad Wolf Girl she had silly amounts of power (Technical Term). She ended the timewar by turning all the surviving Dalek’s to dust (Even though they returned several more times since) And she brought Captain Jack Harkness back from the dead so well she made him immortal. It would have been very easy for her to have topped up the Doc’s energy store, reset it to full. Or even make it never run out. Unlimited Regenerations.

4: River Song

This would be my preferred solution, and the one I would pick myself if i was the writer. In the episode “Let’s Kill Hitler” She poisons the Doctor with a kiss. The poision not only kills within an hour but DNA locks out his ability to regenerate. She then relents and kisses him again, using up all of her future regenerations to save the Doctor. Now it has been revealed in past episodes a Timelord can give up their regeneration energy to other people, but it will leave them basically human and no longer a Timelord (Which happens to River). If I was writing it, I would have the poison have wiped out all the Doctors remaining regenerations, but Rivers energy being available to him. We know she regenerated at least twice, plus power needed to save the Doctor means a possible nine left. +9 Regenerations.

I am not a fan of the idea of giving the doctor unlimited regenerations, it would make him reckless and take out and risks from future episodes. But 9 more? That is a nice number. And it allows for the possible future Valeyard (The Dark future incarnation believed to be between his 12th and final regeneration) And also from the hints given by the Curator (Tom Baker) that in the future the Doctor may look again at old favourite faces. And his implication that he is a future version of the Doctor.

Whatever the writers decide. The Xmas special should be interesting!