The Master was already in his 13th life when he was introduced to the series. His 14th and 15th (and possibly later) incarnations were obtained by taking over the bodies of innocent victims. When the character returned to the show in 2007, the Master was in his 16th (or later) incarnation. Soon, he regenerates into his (~)17th incarnation. Later, he explains that the Time Lords resurrected him to be a great soldier when the Time Lords were fighting the Daleks in the Time War. It is likely that, having been resurrected, he had 12 more regenerations available to him.