Mawdryn Undead.
The limit of 12 regeneration is a biological imperative forced on the Timelords by the Timelords using probably the same technology that enhanced them in the first place, unless this was always a natural ability of their own that Mawdryn reverse engineered their biology into technology.
Thousands, maybe hundreds of thousands of complete Daleks and millions of tons of goopy partial Daleks (numbering into the trillions of distinct consciousnesses?) were given the capacity to regenerate and that capacity was triggered.
It seems like the Doctor handed more than the 12 lives he had left, if he actually does only have just 12 more left.
The limit of 12 regeneration is a biological imperative forced on the Timelords by the Timelords using probably the same technology that enhanced them in the first place, unless this was always a natural ability of their own that Mawdryn reverse engineered their biology into technology.
Thousands, maybe hundreds of thousands of complete Daleks and millions of tons of goopy partial Daleks (numbering into the trillions of distinct consciousnesses?) were given the capacity to regenerate and that capacity was triggered.
It seems like the Doctor handed more than the 12 lives he had left, if he actually does only have just 12 more left.