Huntmaster Liger

• WOTC Official Reminder Textofficial rules
  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.

• WWizards of the Coast2020-04-17
  Official Ruling
  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.
• WWizards of the Coast2020-04-17
  Official Ruling
  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.

Rulings via Scryfall