Tashi, Honored Vow

When Tashi, Honored Vow enters the battlefield,
your commander damage threshold becomes
twenty one plus the times your commander were
casted from the command zone this game and
combat damage dealt by commanders to you is
reduced to zero.
{u}{u}{b}{b}: Add one to the times your commander
were casted from the command zone this game.
Activate this ability any time you could cast
a sorcery, and once per turn.