An ability that triggers "whenever you gain life" triggers just once for each life-gaining event, no matter how much life you gain.
Scion of the Swarm
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Ekaterina Burmak
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
