It depends on how long of trains you run on your layout as well the grades you have. If you run 10 car long trains, and the layout is flat 1 locomotive may be fine. If you run 40 - 50 car trains and have grades on the layout, you may need 3 - 8 locomotives per train, so there are lots of things to consider! I usually run at least 2 enginges on every train, and on unit trains, a mid train and 2 head end engines. Grants sar layout needs pushers to get his big trains up the hill, it makes for very interesting operating sessions!
SD90 makes a great point. My chosen prototype ran trains over helper districts with 20 locomotives in the early 1980's. 15 unit trains were common, and dedicated helper sets of 4-9 units were also common.
True, but you don't really "need" them, you just want them. Nothing wrong with that. If you've got the funds to do it and that's your thing, then that's your decision to make. I guess it's when it gets to the point of people thinking that they "need" those trains, rather than just want them, that it gets hairy. That's the kind of thing that can bankrupt people, split marriages, etc., and that would be a terrible outcome of just trying to enjoy this great hobby. Anyway, that's my 1.2 cents (darn economy).
Shoebox bottom of the closet storage if things are tight on the layout. Rotate as desired. Just make sure YOU are the only one who ever cleans your closet.
