Scion of the Swarm

• WWizards of the Coast2020-09-25
  Official Ruling
  An ability that triggers "whenever you gain life" triggers just once for each life-gaining event, no matter how much life you gain.
• WWizards of the Coast2020-09-25
  Official Ruling
  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.
• WWizards of the Coast2020-09-25
  Official Ruling
  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.
• WWizards of the Coast2020-09-25
  Official Ruling
  In a Two-Headed Giant game, life gained by your teammate won't cause Scion of the Swarm's ability to trigger, even though it caused your team's life total to increase.
• WWizards of the Coast2020-09-25
  Official Ruling
  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, Scion of hte Swarm's ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, planeswalkers, and/or battles at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.
• WWizards of the Coast2020-09-25
  Official Ruling
  The last ability of Scion of the Swarm triggers just once for each life-gaining event, whether it's 1 life from Dazzling Angel or 4 life from Apothecary Stomper.
• WWizards of the Coast2020-09-25
  Official Ruling
  If you gain an amount of life "for each" of something or "equal to the number" of something, that life is gained as one event and Scion of the Swarm's ability triggers only once.
• WWizards of the Coast2020-09-25
  Official Ruling
  In a Two-Headed Giant game, life gained by your teammate won't cause the ability to trigger, even though it caused your team's life total to increase.
• WWizards of the Coast2020-09-25
  Official Ruling
  If you gain an amount of life "for each" of something, that life is gained as one event and the ability will trigger only once.

Rulings via Scryfall