You wouldn't need a VPS for 15-20 players. A VPS would be more suitable for having multiple servers with a large amount of people on both.
A regular game server from a place like Art of War for example, would suit 15-20 players just fine. I've never used AoW personally, but it appears for a 20-slot server it's around $20.
While you could get a decent VPS from NFO for around $25, getting a regular game server would save you the hassle of updates to the OS, VPS security, etc, unless for some reason you really want access to the entire system.
To get the number, look in the file's URL. For that file, you can see from the URL of http://jkhub.org/files/file/1502-luke-skywalker-empire-strikes-back/ that the number is 1502.
Fighter's post in ._. Specifications for a 15 slot server? was marked as the answer