Now it's the case there are no limits on how many times he can regenerate
I remember when I was a little kid watching the episode where they talked about Doctor only being allowed twelve Regenerations (I think it was twelve, I was a kid at the time) and crying over it. I never wanted the Doctor to die and I thought it was mean of the Timelords to tell the Doctor he could only have so many Regenerations before he died. My old man, who was a huge Doctor Who fan himself, pulled me up into his lap and told me this story about how "Regeneration Juice" was very expensive and very hard to make. That in order to ensure they had enough for all the Timelords and their children, and to keep them from fighting over it and use it all up quickly, they they all had to agree to share.
I doubt its Cannon, just a silly little story told to a crying child. But if its got any basis in truth then with there being only one Timelord, and no other Timelords to share with, the "juice" available to the Doctor during Regeneration would be pretty much unlimited.
Or, more realistically, when the RegenValue was originally chosen the BBC didn't have any clue that the show would remain as popular as it has and continue to be produced even now. They probably figured 12, or whatever number was chosen, was safe because they were probably quite convinced the show would have been cancelled before it ever reached that number.