Does a triggered ability that says 'whenever you gain life' trigger once per life gain event or once per point of life gained?

Under CR 119.9, gaining life is treated as a single event regardless of how many points of life are gained. So if a spell or effect causes you to gain 5 life at once, that is one life gain event, and a 'whenever you gain life' trigger fires exactly once for that event.

Many such triggered abilities are also written to capture the amount gained — for example, 'whenever you gain life, you get X energy counters, where X is the amount of life gained.' This phrasing lets the ability scale with the quantity, but it still triggers only once per event (CR 603.2).

The key distinction is between a single event granting multiple points versus multiple separate events. If two different effects each cause you to gain life independently in the same turn, each is its own event, and 'whenever you gain life' triggers separately for each one.

Concrete example: You cast Revitalize, which reads 'You gain 3 life.' This is one life gain event, so a card like Heliod, Sun-Crowned that says 'whenever you gain life, put a +1/+1 counter on target creature or enchantment' triggers exactly once — you place one counter, not three.

This is also why effects like Dina, Soul Steeper ('whenever you gain life, each opponent loses 1 life') drain each opponent for 1 life total from a single Revitalize, not 3 — because the trigger fires once (CR 119.9, 603.2).