How many times can Capaldi's Doctor regenerate, post-S9E2?

Question

During Doctor Who S9E2, The Witch's Familiar,

The Doctor gives away enough of his regeneration energy to revive Davros and every Dalek on Skaro, including all the "dead" ones.

Capaldi was the first regeneration of a new cycle, but surely this event shook things up. How many more Doctors will we have before Moffat has to get creative again?

The events of S9E2 give me a reason to think there would be a different answer than the one to How many regenerations does the Doctor have after The Time of the Doctor?

Answer 1

In-universe, we don't know. Exactly what the Time Lords gave him in Time of the Doctor has never been made completely clear: it could be anything from one, to thirteen, to infinitely many new regenerations. The Doctor himself doesn't know, as he said in Kill the Moon in series 8:

You'll have to spend a lot of time shooting me because I will keep on regenerating. In fact, I'm not entirely sure that I won't keep on regenerating for ever.

In fact, in series 9 episode 12, Hell Bent, we discover that

neither does Rassilon, the Time Lord in charge of giving him the extra regenerations.

From the transcript:

RASSILON: A direct order of your President! You leave me no choice.
(Rassilon activates his gauntlet.)
RASSILON: How many regenerations did we grant you? I've got all night.

Out of universe, the answer is obviously for as long as the show continues, or if you want to be cynical, for as long as it continues to make money. Doctor Who will continue for at least another 5 years and maybe another 50 years. New actors will be cast; new plot twists will be invented if necessary. Nothing is set in stone in a series about time travel!

Answer 2

It rather hangs on the details of the regenerative process, which we don't know. If the process requires a specific amount "x" of the energy, and a Time Lord only has x*n amount of said energy, than conceivably the energy he's shared over the recent years has reduced the number of possible regenerations by one or two.

If, however, he has access to an unlimited amount of the energy, but can only use it to regenerate a set number of times, then the times he's tapped into it doesn't count towards than number. He loses the ability to tap into that energy when his regeneration cycle ends.

I'm leaning towards the second scenario. We've seen that after a regeneration, a Time Lord has quite a bit of extra energy left over to make "minor fixes" like restore a hand, change their hairstyle, or even completely change their body again.

The Doctor only used a portion of the energy of a regeneration after being hit by a Dalek, dumping the rest into his severed hand, but as we learned later, that "counted" as a regeneration. So that adds weight to the idea that it's the process of the tapping into the power if what counts, and not the power used.

Nine's regeneration was just a light show, Ten's spat out enough energy to almost destroy the TARDIS control room, and Eleven used up enough energy to destroy a Dalek fleet, and still had enough for a full regeneration. I'd hazard a guess that the amount of energy expended against said fleet in Time of the Doctor exceeds the amount siphoned from him in Witch's Familiar.

River allegedly used up "all of her regenerations" (as opposed to all of her regeneration energy) to heal The Doctor in Let's Kill Hitler. But since her connection to the powers and abilities is under question, her process may change. Perhaps a better description is she burned out her ability to access the energy.

Answer 3

It is likely a finite amount of energy, I don't think that even a Time Lord body could hold an infinite amount of energy. I like to believe the "packets" of regeneration energy is canon, that each Time Lord has twelve small organs inside their body, or twelve "packets" inside one organ, filled with enough energy for a full regeneration. I also like to think the Time Lords gave 11 a little extra juice to get rid of the Daleks at that point, but that that effect would not be possible at any other time.

It's still difficult to say, he said the energy to heal Davros would likely use up some, but not all, of a full regeneration, but they took more energy than he had expected, so that took up at least 1, maybe 2. Then there is the episode "Lie of the Land", where he must have surely used a little bit as a demonstration, either that or some hidden technology. Probably actually makes more sense for the technology route, as why else would he use some? So I'd say he's got 10 left, since he made it known he was aware of Davros's plan, so gave him enough for his plans, but nothing ridiculous.