Deathwatch offers rules to fight and attack with hordes or large groups of enemies.

I am running Final Testament right now and the mission mentions "hordes" of enemies. Are there any comprehensive rules in place to fight large groups of enemies? I think I am missing it if there is.

Or are we (GMs) supposed to roll for each individual enemy against the PCs?

See Enemies of the Imperium Chapter 5, Veterans of the Spinward Front. It has rules for formations, which I use to organise infantry into appropriately sized squads.

Lovely. Thanks a lot, Sgt!

I knew I was missing something.