Yes, you can run one with as many as you wish, but generally they support a maximum of only 254. ;-)with ONLY 1 computer?
You can even configure them to support much more than that, but you will, of course, run into bandwidth issues.
By using one, you'll shield your computer from many of the attacks via the internet.
BTW, many of those boxes run Linux or BSD Unix, because those operating systems, unlike Windows, are very secure on their own. Macs are BSD based. Many people, myself included, build their own firewall/router with Linux on an old computer. You can even get a very secure version of Linux developed by the U.S. National Security Agency.