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.

Gluttonous Hellkite's triggered ability will resolve before Gluttonous Hellkite does. If Gluttonous Hellkite is countered or otherwise leaves the stack in response to that triggered ability, the triggered ability will still resolve as normal.