The basics are: a set requirement of clubs. Usually at least 10-13 with the driver as a requirement. Some have other requirements like putters or wedges as well.
Players can fill in the rest however they like.
Back in the day, there used to be a bit more piece by piece deals. Meaning woods with one company, but irons with another. Those deals are basically all gone. Companies across the board have all gone to a full line as they call it. Drivers are almost always a must as it is usually their most marketed product.