To determine "the number of creatures in your party," check whether you control a Cleric, whether you control a Rogue, whether you control a Warrior, and whether you control a Wizard. The number is the total number of those checks to which you answered yes. Each creature you control can be counted for only one of those checks.

If a creature you control is dealt lethal damage at the same time that you gain life, it can't receive a counter from The Destined White Mage's second ability in time to save it.

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

If a creature has more than one party creature type, and there are multiple ways to count that creature that could result in a different number of creatures in your party, the highest such number is used. For example, if you control a Cleric and a Cleric Wizard, the number of creatures in your party is two. You can't choose to have it be just one by counting the Cleric Wizard first as a Cleric.

The Destined White Mage'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.

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 Destined White Mage's third ability triggers only once.

Some cards refer to you having a "full party." This is true if the number of creatures in your party is four.

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, The Destined White Mage's third 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.