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 (unless it's attacking and somehow gains trample). Its ability won't trigger a second time that combat.

The ability triggers just once for each life-gaining event, no matter how much life was gained.

If multiple creatures with lifelink you control deal combat damage at the same time, the damage dealt by each of those creatures is a separate life-gaining event and Drogskol Reaver's ability will trigger that many times.

If a creature with lifelink you control deals damage to multiple creatures, players, and/or planeswalkers at the same time, the ability will trigger just once. The damage dealt by that creature is a single life-gaining event.