Scion of the Swarm

An ability that triggers "whenever you gain life" triggers just once for each life-gaining event, no matter how much life you gain.

If Scion of the Swarm is dealt lethal damage at the same time that you gain life, it won't receive a counter from its ability in time to save it.

Each creature with lifelink dealing combat damage causes a separate life-gaining 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.