1. When on your server, right click the highest channel and select Edit Virtual Server

2. A new window will appear. Then, in the top section you will see the option to set the amount of reserved slots.
NOTE: Regular users assigned in the Guest group will not be able to occupy a reserved slot. That is, if a server is 18/20 full with 2 assigned reserve slots users (except Admins) will not be able to connect.

3. After you have made your changes, click Apply and the changes will be made.
