Originally Posted by The Alpha Gamer:
“If 10.5 counts as a regeneration then surely so should 11 healing River in TATM.”
The difference might be in the quantity of energy released. Though being able to heal others isn't a skill the Doctor exhibited before he was healed by River. Perhaps it's a bit like a gall bladder, some organ collects regenergy which means that timelords naturally have a small amount in their bodies at all times. In a crisis that organ releases the built up energy causing, under normal circumstances, a complete rebuild of the body. Choices then are either there's a single organ which can only be used 12 times or there are 12 and only one can be filled at a time. Either explanation would mean that it could take time after one regeneration to restock for the next.
An alternate explanation would be that instead of there being a store of energy it's a store of some enzyme that neutralises what might be a nanite swarm that gets released when a regeneration is complete. That would allow a massive dose of an artificially created form of that to prevent regeneration. It would also mean that if you ran out of that then you'd regenerate to death or possibly into the state the Master was in in Deadly Assassin.
To be honest a bit of ambiguity allows good stories to be written and as long as any inconsistencies aren't too apparent, important or are actually part of the plot then it's probably best we don't know everything. Despite the impression he gives the Doctor certainly doesn't know everything and that allows him to get round the rule about being unable to rewrite history. It's also why he has an aversion to being at some events such as his own grave.