Does lifelink from a source that deals damage to multiple creatures trigger once or multiple times?

Lifelink is a static ability, not a triggered ability. It does not use the stack and never 'triggers.' Instead, whenever a source with lifelink deals damage, that same damage event causes its controller to gain that much life as a replacement-style continuous effect (CR 702.15b). There is no separate trigger that goes on the stack.

When a single source with lifelink deals damage to multiple creatures at the same time — for example, via trample, a fight, or simply being blocked by multiple creatures — all of that damage is dealt in one damage-dealing event. The controller gains life equal to the total damage dealt in that event, all at once (CR 119.3f).

Concrete example: You attack with a 5/5 with lifelink that is blocked by two 2/2 creatures. It deals 2 damage to each blocker (4 damage total). You gain 4 life in one simultaneous life-gain event — not two separate 2-life gains.

This distinction matters for effects that care about 'whenever you gain life' triggered abilities (like Ajani's Pridemate). Because lifelink produces a single life-gain event from one damage event, such a triggered ability would trigger only once, seeing a total gain of 4, not twice for 2 each.

Relevant rules: CR 702.15b (lifelink definition as static ability), CR 119.3f (lifelink and life gain timing), CR 702.15a (lifelink is a static ability, not triggered).