The simple answer is 'We don't know'.
The first complication arises in "The Keeper of Traken", as the Master took over the body of Tremas(played by Anthony Ainley). There is disagreement among fans as to whether this counts as a fourteenth body or not. While it is a new body, it as not a regeneration. Neither was the body of Bruce(played by Eric Roberts) in "Doctor Who".
In the New Series, the Master appeared played by Derek Jacobi(in "Utopia"), which is definitely a new body, we just don't know how many bodies it is on from Delgado's Master. The Jacobi Master then regenerated into a new body, played by John Simm. Another problem emerges in that Simm's last television appearances were in "The End of Time", where the Master stated that this incarnation was not the same incarnation as his previous 'Harold Saxon'(the alias of the Master when he first regenerated into Simm) incarnation. He did not regenerate between these two appearances. Instead, his body was cremated (at the end of "Last of the Time Lords") and a new, but outwardly identical, body was created by his resurrection in "The End of Time".
Spin-off media confuses the issue further. In the Doctor Who Magazine comic "The Fallen" (comic story) the Master has taken over the body of a homeless man in the same way that he had previously taken over Bruce's body in The TV Movie.
The Big Finish Audio "UNIT:Dominion" (audio story) introduces a Master played by Alex MacQueen. it is revealed that he is a Master from after The TV Movie who the Time Lords have revived and given a new regenerative cycle to. He is thus before Derek Jacobi. but again we don't know how many incarnations separate these two incarnations of the Master.
Finally the Virgin novel "First Frontier" (novel) confuses issues further by featuring the Ainley Master regenerating into an incarnation described as resembling the actor Basil Rathbone. This 'Basil Rathbone Master' appears throughout the Virgin new Adventures, yet these developments are completely ignored, and totally contradicted by all other media.