show (you can't keep a good Who villain down!)

 Apologies for the masculine gender. It was partly to avoid spoilers for those who haven't seen Missy yet, and partly because "he/she" sounds clumsy and he has been male almost every time.

A:

All of them, and then some
The Fourth Doctor episode "The Deadly Assassin" features the Master at the end of his regeneration cycle (emphasis mine):

Spandrell: It seems clear how it happened. The Master tried to trap the Doctor in the APC net by overloading the neuron fields. Then he collapsed and died, leaving Chancellor Goth still connected into the circuit. 
Borusa: Natural causes? 
Spandrell: Yes, sir. His body was extremely emaciated. He had come to the end of his regeneration cycle.
Doctor Who Season 14 Episode 3: "The Deadly Assassin"

Hegot better, of course, and circumvented the limit by hijacking someone else's body. In "The Five Doctors", the High Council of Gallifrey offered the Master a brand new regeneration cycle, but it's not clear whether or not they delivered on that promise:

Master: What makes you think I want your forgiveness? 
Castellan: We can offer you an alternative to your renegade existence. 
Borusa: Regeneration. A complete new life cycle. 
Master: What must I do? 
Borusa: Rescue the Doctor. 
Doctor Who "The Five Doctors" (1983)

And, of course, he's been explicitly brought back to life at least twice, with an unknown number of regenerations earned each resurrection:

By the Time Lords to fight in the Time War:

Master: The Time Lords only resurrected me because they knew I'd be the perfect warrior for