Celestial Unicorn — ruling 1

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.

Celestial Unicorn — ruling 2

If Celestial Unicorn 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.

Celestial Unicorn — ruling 3

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.

Celestial Unicorn — ruling 4

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.

Celestial Unicorn — ruling 5

In a Two-Headed Giant game, life gained by your teammate won't cause Celestial Unicorn's ability to trigger, even though it caused your team's life total to increase.

Celestial Unicorn : règles, fonctionnement et interactions (MTG)

Celestial Unicorn is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever you gain life, put a +1/+1 counter on this creature.

How it works : Celestial Unicorn

Original English text — Wizards of the Coast / Scryfall

#1July 23, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#2July 23, 2021· Wizards of the Coast
If Celestial Unicorn 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.
No other card is named in this ruling.
#3July 23, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#4July 23, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#5July 23, 2021· Wizards of the Coast
In a Two-Headed Giant game, life gained by your teammate won't cause Celestial Unicorn's ability to trigger, even though it caused your team's life total to increase.
No other card is named in this ruling.