13 Is An Unlucky Number
According to canon, a Time lord can only regenerate 12 times.
The thirteenth Doctor stood alone at the centre of the TARDIS. A tear rolled down his cheek. The Master had found a way to cheat death, but much as the Doctor had tried, he hadn't been able to figure out a way. He glanced down at his chest, at the life blood seeping out of him, pooling by his feet. His face was pale, damp with perspiration. He collapsed forwards, hands gripping at the controls. He could feel the raw energy pulsing through the TARDIS, and cried out one final time as the energy entered his body.
Two heartbeats ended.