If Drogskol Reaver deals enough first-strike damage to destroy each creature it's blocking or was blocked by, it won't deal any damage during the regular combat damage step and its last ability won't trigger a second time that combat. This is different if Drogskol Reaver is attacking and somehow gains trample.

Drogskol Reaver's last ability triggers just once for each life-gaining event, whether it's 1 life from Dazzling Angel or 4 life from Apothecary Stomper.

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 last ability of Drogskol Reaver 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, Drogskol Reaver'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.