The two feed stores around here would let you buy one chick if you wanted, but that would be bad (for the solo chick). No minimum.
And, apparently, no maximum, either. If the chicks of a particular breed sell out, they're just out of 'em until the next chick delivery day. My favorite feed store has at least 10 troughs of chicks; hens and roosters separated, and a couple of straight run troughs. If there is an obvious difference between breeds at the chick stage, they may have two different breeds in the same trough.
If I had property of my own, I could easily walk in and purchase hundreds of chicks on a single visit.
But most of the fun is selecting which chick or two I want out of this or that trough.