The smallest collection comprises three items.
f you have one of a type of item, that’s just a possession; if you have two, then you have a “pair” of them or a “brace” of them (e.g., a brace of dueling pistols); at three, the word is “collection.” I suppose the idea is that we have a common word for a pair of items (there, I’ve just used it), but no common word for three items of a kind, and we switch to “collection.” This must be ancient, since it is in effect, “one, two, many.” We do have “a few” (3-4, IMO) and “several” (5 or 6, 7 at the most, IMO).
Of course, you can have a collection of collections (e.g., you collect card decks (a deck being a collection of cards) or jigsaw puzzles (each puzzle a collection of pieces). (Obviously, you can also have a collection of pairs, as for those who collect salt-and-pepper sets.)
Note, however, that we are restricting ourselves to instances in which the components are of individual interest: a salt shaker does not represent a “collection” of salt. For aggregates of indistinguishable items, we might use, say, “a bucket of x” or “a bag of x,” but we would never say “a collection of x.” A collection of chess sets, however, is a collection of sets (a tip-off word) that are collections of chess pieces.
Let’s define collection depth as the maximum number of levels of a collection (since some collections within a collection may not be a complete collection—a card deck missing some cards or a jigsaw puzzle missing some pieces).
A collection of shaving brushes: collection depth = 1
A collection of straight razors that includes one or more 7-day sets: collection depth = 2
A collection of 20-pack cartons or carts of packages of five blades each—e.g., I have cartons of Astra Superior Platinum and Wilkinson Sword blades, and cards containing 20 packs of Gillette Silver Blues, Zorriks, and others, and each of the packs is a collection of 5 blades. Collection depth = 3
Any ideas for collection depth = 4? It need not be shaving related, but cool if you can do it.