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"
He got 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 a Time War.
Doctor Who Series 3 Episode 12: "The Sound of Drums"
By the nutcases who brought him back in "The End of Time":
Governor: Mrs Saxon. Let me introduce myself. I'm your new Governor. I'm afraid the previous Governor met with something of an accident, which took quite some time to arrange. Miss Trefusis, if you will prepare. You kept your silence well, Mrs Saxon. Your trial was held in secret, with no jury, so no one knows who Harold Saxon was, where he came from, why you killed him. Make her kneel. There are those of us who never lost faith. And in his wisdom, Harold Saxon prepared for this moment. He knew that he might die and he made us ready. Tonight, Mrs. Saxon, he returns.
Master [OC]: [maniacal laugh]
(Miss Trefusis holds out the Master's signet ring. Lucy gasps.
Meanwhile, Wilf is looking out at the kitchen window at the storm while the Woman in White briefly appears on the TV screen in the other room.)
Governor: As it was written in the Secret Books of Saxon, these are the Potions of Life.
Doctor Who "The End of Time Part 1"
How many he has left at this point is really anyone's guess; considering how infrequently we see him, every change of actor could have an untold number of intermediate regenerations.
All told, we know he's used at least 14:
- The original 12, ending with Pratt/Beevers
- Jacobi to Simm
Simm to Gomez (off-screen)
Of the other actors to portray the Master, it's never 100% clear whether he regenerated into that new body, or simply aged into it:
- William Hughes played the young Master in the flashback in "The Sound of Drums" and "The End of Time"; that was probably his first body
- Roger Delgado played the Master during the Third Doctor's era. We don't know if the Master regenerated into the Delgado body, or if William Hughes grew up into Roger Delgado
- Peter Pratt and Geoffrey Beevers both played the Master at the end of his regeneration cycle; these are implied to be the same body. Once again, we don't know if there were any intermediate regenerations
- Anthony Ainley played the role for the last eight years of Classic Who, but he wasn't a regeneration; his was a body the Master stole
- Likewise with Eric Roberts, from the Movie