@eshep i think you should stick with one (or two, with hardware.nix) until that gets awkward.
Single-file config only starts to get awkward when you're trying to configure more than one machine in a single Nix config. It can also get awkward when you start using home-manager and you want to configure more than one user.
But if you don't want to do either of those things, just keep it in a single file until you have an obvious reason not to.