If you gain or lose an amount of life “for each” of something, that life is gained or lost as one event and Wax-Wane Witness’s last ability triggers only once.

In a Two-Headed Giant game, life gained by your teammate won’t cause Wax-Wane Witness’s last ability to trigger even though it caused your team’s life total to increase.

Wax-Wane Witness’s last ability triggers just once for each life-gaining or life-losing event, no matter how much life was gained or lost.

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, Wax-Wane Witness’s last ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.