Eleven pieces that go together in more than one direction. The test is simple and most rails fail it.

The test: hang everything on one rail, and check that any top works with any bottom, in any order, without a single combination that needs explaining. Eleven pieces gives you a lot of mornings. It also takes about four attempts a season to get there, and the piece that breaks it is almost always the one everybody liked most in the buying room.
What it is not is a capsule wardrobe in the magazine sense — a navy blazer, a white shirt, and an instruction to buy quality. Nobody needs to be told to buy quality. What people need is the specific, boring information about which trouser goes over which boot, and that only comes from hanging it up and looking at it.
It is bought as pieces, not as a set, and most people take three or four of them. The point of hanging it as eleven is that you can see what you are choosing against — which is impossible on a shop floor organised by fabric, and impossible online full stop.
If you are starting one from nothing — new job, new city, a body that has changed — book a wardrobe audit before you buy anything. Two hours in your own home, and the list that comes out of it is usually shorter and duller than this page, which is the point.