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 Minwu's last ability triggers only once.

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

Minwu's last ability triggers just once for each life-gaining event, whether it's 1 life from Al Bhed Salvagers or 3 life from Balamb T-Rexaur.

If Minwu or another Cleric you control is dealt lethal damage at the same time that you gain life, it will die before Minwu's last ability would resolve.

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