Does lifelink from multiple creatures stack or does it only trigger once?

Lifelink is a static ability that causes life gain to occur as a consequence of damage being dealt, not a triggered ability that fires once per combat. Because it is handled as a continuous effect (not a trigger), there is no 'stacking' issue in the traditional sense — each source simply applies independently. (CR 702.15a, 702.15b)

Whenever a creature with lifelink deals damage, you gain that much life. If you control multiple creatures with lifelink and they all deal damage, each one causes you to gain life equal to the damage it dealt — completely separately. The amounts are not combined into a single event; each creature's damage is its own life-gain event. (CR 702.15b)

This matters for interactions like 'Whenever you gain life' triggers — each lifelink damage event from a different creature will trigger such an ability separately, potentially multiple times in a single combat step.

Example: You attack with a 2/2 lifelink creature and a 3/3 lifelink creature, both unblocked. When damage is dealt, you gain 2 life from the first creature and 3 life from the second creature — 5 life total. If you control a card that triggers 'whenever you gain life,' it triggers twice.

If a single creature has lifelink from multiple sources (e.g., granted by two different effects), it still only deals damage once, so you only gain life once for that damage event. Having redundant lifelink on the same creature has no additional benefit. (CR 702.15a)