Each creature with lifelink dealing combat damage causes a separate life gain event. For example, if two creatures you control with lifelink deal combat damage at the same time, you will gain 2 additional life from Cleric Class's first ability and its level 2 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), you will only gain 1 additional life and the ability will trigger only once.