Use the number of creatures that were actually sacrificed this way, not the value of X, to determine the number of +1/+1 counters with which Gluttonous Hellkite should enter the battlefield
Disney Lorcana The Disney Lorcana TCG website offers a bunch of helpful resources for learning the game, including a quick-start rulebook that can be downloaded at the bottom of this page
(Each affected player or team gets one of those life totals back.) You choose which player or team gets which life total when the ability resolves
If you can't legally choose a mode because all three have been chosen that turn, that instance of the ability is removed from the stack with no effect
The main commanders I see being popular this set are Hinata, Isshin, and Satsuki
Whenever Baldin attacks, up to one hundred target creatures each get +0/+X until end of turn, where X is the number of cards in your hand