In one of the old
old episodes it said he only had 13....
"The BBC's Series 4 FAQ suggests that now the Time Lord social order has been destroyed, the Doctor may be able to circumvent the limit on regenerations; it says: "Now that his people are gone, who knows? Time Lords used to have 13 lives." In Death of the Doctor (a 2010 The Sarah Jane Adventures serial), the Eleventh Doctor responds to a question from Clyde Langer by saying he can regenerate "507" times. Early news reports, before the episode was broadcast, suggested he would say there is no limit to the number of regenerations. Writer Russell T Davies explained in an interview with SFX that the line was not intended to be taken seriously and is instead a commentary. He insisted that the "thirteen lives" rule was too deeply entrenched in the viewer consciousness for his throwaway line to affect it."
They'll probably come up with some clever way to say, Oh, hey, looky here! I still have more regenerations and it makes sense-ish.