how many in a collection, well that depends on what you want to collect
take the enfield rifle, a modest collection is about a dozen. That will give you a nice little sample of a broad crossection. Now a good collection your looking at at least 40, then you can start to specialize, say LongBranch production. Don't forget to add a few sporters like the Parker Hales and the EAL (civi version) into the collection. Now a serious collection of enfields is well over 100. Nagant rifles are similar.
Pistols I don't really collect but 1911's, Tokarovs, Hi-Powers, Lugers, those all have many different varients, a collection can get huge fast.
Or you can try to collect a sampling of WW2 sidearms, your still looking at over a dozen just for starters.
So it all depends on what your trying to acheive with your collection.
Myself I have a little shooting collection, with a few WW2 Canadian pistols lumped in.