Rulings

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.
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.