The 13th regeneration is the final and fatal one. Apparently when that level is reached they age rapidly physically and deteriorate as the Master did , however new regenerations can be added to a timelord by the Council on Gallifrey. Case in point the Master was ressurected to be a soldier on the frontlines of the last time war but ran away and hid as Proffesor Yana. So he was given at least one regeneration as he used it in the TARDIS before he stole it ... and boobytrapped it.
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