The value of X is determined only as the triggered ability resolves. Once that happens, the value of X won’t change later in the turn even if Huntmaster Liger mutates again, although in that case the creatures will receive a second, larger +X/+X in addition to the one generated by the ability’s earlier resolution.

Huntmaster Liger’s triggered ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won’t get +X/+X.