Whenever you gain life, each opponent loses 1 life.
The ability of Epicure of Blood triggers just once for each life-gaining event, whether it's 1 life from Impassioned Orator or 4 life from Dawning Angel. If you gain an amount of life "for each" of something or "equal to the number" of something, that life is gained as one event and the ability of Epicure of Blood triggers only once.
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, Epicure of Blood's 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.
In a Two-Headed Giant game, life gained by your teammate won't cause the ability to trigger, even though it caused your team's life total to increase.
In a Two-Headed Giant game, Epicure of Blood's ability causes the opposing team to lose 1 life twice.
