From wiki (with regards the five doctors).....
This is the first time it is suggested that a new cycle of regenerations can be bestowed on a person (in this case the Master), implying that it could be possible to circumvent the twelve-regeneration limit established in
The Deadly Assassin. However, the Master is occupying a non-Time Lord body, so whether this can be applied to a Time Lord who has already reached his thirteenth incarnation is unclear. Years later, however, the episode "
Utopia" shows the Master regenerating and in the following episode "
The Sound of Drums" indicates that he had been "resurrected" (the Master's own word, left unexplained) by the Time Lords to fight in the
Time War, suggesting a new regeneration cycle was indeed bestowed upon him.