If Aerith Gainsborough is dealt lethal damage at the same time that you gain life, it will die before its second ability would resolve. Its last ability will use the number of counters that were on it when it was last on the battlefield.

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 Aerith Gainsborough's second ability triggers only once.

In the rare case where enough -1/-1 counters are put on Aerith Gainsborough at the same time to make its toughness 0 or less, the number of +1/+1 counters on it before it got those -1/-1 counters will be used to determine the value of X in Aerith Gainsborough's third ability. For example, if there are two +1/+1 counters on Aerith Gainsborough and it gets three -1/-1 counters, you'll put two +1/+1 counters on each legendary creature you control when Aerith Gainsborough's third ability resolves.

Aerith Gainsborough's second 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.

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

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, Aerith Gainsborough's second 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.