A Time Lord was typically allowed 12 regenerations in the classic Doctor Who continuity.
That limit means a standard Time Lord could have 13 incarnations in a single regenerative cycle: the original body plus 12 replacement bodies. The rule was first established on television in The Deadly Assassin, broadcast in 1976 during the Fourth Doctor's era. It transformed regeneration from a mysterious way to recast the lead actor into a defined piece of Time Lord biology and law.
The distinction between regenerations and incarnations causes many quiz mistakes. “Twelve regenerations” does not mean twelve total bodies; it means twelve changes after the first body. The Doctor's apparent limit became a major story problem as the programme approached the thirteenth incarnation.
Classic-series stories also show that the rule was not absolutely impossible to bypass. In The Five Doctors, the Time Lords offer the Master a new cycle in exchange for his help. Later continuity gives the Doctor another cycle in The Time of the Doctor. Those exceptions do not change the classic-series norm: 12 regenerations, producing 13 lives in the usual cycle.