Celestial Unicorn

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 Celestial Unicorn's ability triggers only once.

The ability of Celestial Unicorn triggers just once for each life-gaining event, whether it's 1 life from Dazzling Angel or 4 life from Apothecary Stomper.

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, Ajani's Pridemate'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.