The web page says:
Single User License: A Single User license gives you the right to use SL4NT on one computer.
This means that you need only one "user" license.
You are correct in that the "user" license should be really called a "server" license. It was named this way because more than 10 years ago, SL4NT was sold through the online reseller platform Kagi, which offered only three kinds of licenses: user, site, world-wide.