2021-07-23 : An ability that triggers “whenever you gain life” triggers just once for each life gain event, no matter how much life you gain.

2021-07-23 : 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.

2021-07-23 : 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.

2021-07-23 : If you gain life at the same time Celestial Unicorn is dealt lethal damage, it dies before its ability can put a +1/+1 counter on it.