Each creature with lifelink dealing combat damage causes a separate life gain event. For example, if two creatures you control with lifelink deal combat damage at the same time, a "whenever you gain life" ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

Dina's first ability triggers only once for each life gain event, no matter how much life was gained.

When Dina's first ability resolves, each opponent loses only 1 life, no matter how much life was gained.

X is the power the creature had when it was on the battlefield, not the power it has in the graveyard.