The death of the Master's eleventh incarnation was acknowledged by Missy, herself a product of the Master's eighteenth death, on her Spacebook profile. (PROSE: Girl Power!)
Nature
By their nature as a Time Lord, the Master was born with an initial life cycle of 13 lives, which the Eighth Doctor knew had all been used by the time of the Old Master's trial on Skaro. (TV: Doctor Who) Missy acknowledged that she had first died thirteen times before before taking over "some bloke's body". (PROSE: Girl Power!) In fact, the Decayed Master, who claimed to be nearing the end of his "twelfth regeneration", had acquired "a new body" in the Trakenite Tremas. (TV: The Keeper of Traken) Surviving his execution on Skaro as a Deathworm Morphant, the Old Master took the body of Bruce, a human. (TV: Doctor Who)
The Bruce Master claimed to Chang Lee that the Doctor had taken most of his regenerations whilst the Doctor told Grace Holloway that the Master was both "on his last life" and had "[ran] out of all his lives". In their ensuing confrontation, the Master claimed that it was because of the Doctor that he had "wasted all [his] lives". The Doctor challenged this, reminding Lee that the Master claimed that he had stolen those lives when the reality was that the Master sought the Doctor's remaining lives after having "used all his lives", noting that the Master himself had just admitted to wasting "all of them". (TV: Doctor Who) In any case, the bodily theft was followed by the Master's fourteenth death. (PROSE: Girl Power!)
Behind the scenes
Invalid sources
The Doctor Who Role Playing Game by FASA, which admits to taking liberties with the source material in its opening pages, gives a rundown of the Master's first thirteen incarnations in "The Master" supplement book, which was similar to (but not entirely consistent with) the in-universe biography given for the Master in FASA's own CIA File Extracts.
According to the book, the Master could control the form of his incarnations, and frequently used the same face. In a constant, bearded aristocrat form, the Master worked as a researcher on Gallifrey until his fifth incarnation attempted to lead a rebellion on Gallifrey, with the War Chief among his followers, which ended in failure and forced the now renegade to regenerate into an "average" clean-shaven form, becoming known as the Monk in his sixth and seventh incarnations. When the Master's activities as the Monk became known, he chose a new disguise when a crisis triggered his next regeneration, assuming a bearded appearance similar but distinct to his initial form. The appearance by which he would become best known, the Master was a dark, strikingly handsome and apparently middle-aged man, whilst in reality he was over 800 years old. He had an average height and build with distinguishing features including a "satanic" beard and piercing grey eyes whilst he was generally dressed in a black tunic with gloves.