Means there’s a lot of a thing.

Pack is fairly obvious.

“A whole host of reasons” is a fairly common phrase to mean someone has a lot of reasons for a decision.

“Sea” is a bit less common, but you might say something like “he looked down upon a sea of friendly faces”.

“Drove” is honestly one I would consider wrong, but only on a technicality. I would consider this sense of drove to be a “lost singular”, in the way that “evitable” is a lost positive (from the real negative “inevitable”) or “whelmed” is a lost form from under- and overwhelmed. You never see “a drove of” something, but you do see something “in droves”.