Normally, Gallifreyans only have 13 lives.
It's possible that this is not a biological or physical limit, but was imposed by the Time Lords. There's a lot of circumstantial evidence against this (e.g., the fixed amount of regeneration energy mentioned in "Mawdryn Undead" and "Let's Kill Hitler"), but no absolute proof that it's not true.
Even if it is a fixed limit, there must be ways around it. The Time Lords were able to resurrect the Master and give him a new cycle of regenerations, and apparently did something similar with Rassilon.
The Doctor did get a new set of regenerations in "The Day of the Doctor"