if you count all the doctors then they have gone over it already (I mean include stuff like the US TV series that never went any where and stuff like that) On the other hand we also don't know how many regenerations the doctor had before the TV show started (is it fair to assume he did not have any regenerations in his life before that?) On the other hand we know that the master regenerated more then his 12 times so it is not a cannon (biological rule of the species)