If your server has two or more slots less than you were expecting it to it's likely you have spectators or hackers (maybe both) enabled on your server.

To disable hackers you must go into the startup.txt file in ClanForge (available in the configs submenu of the edit profile section) and find:

vars.Hacker "true"

simply change "true" to "false" and you'll gain two of your slots back!

To disable spectators go that that same file and add the line:

vars.maxSpectators "0"

This will ensure spectators have been disabled, and you should now be up to your full slot count.

To apply these changes it's as simple as hitting the big orange "Save&Apply" button at the top or bottom the submenu. (This will restart your server, if you'd like to wait until it's next restart before enabling these settings just hit the green "Save" button)