15
don't confuse FTP accounts and XOOPS users.
FTP (File Transfer Protocol) is for you to upload/download files to and from the webserver and you only need one for yourself. Anymore would be, if you have others, who should be able to upload files to a certain area of the server. This is NOT for the casual visitor or XOOPS user.
The amount of users, your XOOPS installation can have is only limited by the MySQL database... and it will take many thousand users before you reach any limit there, I'd say.