It’s never really been quantified how regeneration works. People kind of assume there’s an objective limit, like he’s got so many magic potions in his inventory. Use one, and it’s gone. I dunno. That only makes sense as a plot device. Which, granted, is exactly what regeneration is. Still, no need to be that blatant.

The way I’ve taken it is that each regeneration is kind of like a mutation. You can only go so far before your system gets so screwed up that any further change is a humongous risk at best.

“The End of the World” implies that Eccleston has nine different DNA strands in him. Carry that forward, and I can imagine how that might get tangled after a certain point.

In that case the limit isn’t based in some kind of volume of opportunities for the spending so much as in the consequences of having regenerated.

There are other interpretations. Perhaps the cells of each incarnation are sort of impregnated with a certain amount of energy, that can be set off for regeneration. Once it’s spent, it’s spent. Let’s say that, as of “Journey’s End”, Tennant has now half-regenerated. To finish regenerating, perhaps he needs a kick-start from Donna, as he has already blown his proverbial wad. Which will leave each of his next two incarnations with a new set of cells, with their own preset regeneration bombs. As it were. Thus presently bringing him to thirteen.

Maybe there’s something kind of like super-mitochondria in a Time Lord cell. From my understanding, in a human cell the mitochondrion acts as a power source. If there were a similar sort of organelle in a Time Lord cell, that in an emergency were to rupture, releasing a certain potent energy, intended to completely reform the cell — well, that might work. The reformed cell would, naturally enough, have its own version of that organelle.

Actually, looking up mitochondria on Wikipedia — they deal with cell death, control of the cell cycle and cell growth… Yeah.

If we were to assume something like that, then Tennant will have perhaps wasted all that energy without it making over his cells — so therefore he can’t regenerate again until he has been regenerated. Which would still leave his future incarnations free to regenerate, as they would have fresh cells, with their own regeneration-energy organelles.