I'll agree with the others here - if all the computers run to one place, a single switch works. If you need multiple locations, then as many switches as you have locations.
You could use a network device, even including the main router to filter traffic in some fashion - and those who run large networks might recommend that - but it's added complexity with little benefit for a small shop. Also, it's a bandwidth bottle-neck. Even if you put the servers on a different subnet/vlan than all the computers, while it's true that you could control traffic a lot better, you're now forcing all traffic down a single pipe to the router and then back to the switch to the server. With direct switching, you could get full gigabit speeds; with a router in the middle, you're limited by what it can process. If you're concerned about what types of traffic can hit the server, then use the internal firewall to limit what's open.