Huntmaster Liger
King Caesar, Ancient Guardian
Creature — Cat
Mutate 
(If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Whenever this creature mutates, other creatures you control get +X/+X until end of turn, where X is the number of times this creature has mutated.
Illustrated by
Ikoria: Lair of Behemoths#370UncommonEnglishLegality
Rulings
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.
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.