There is a reply to the topic on the steam forums.
Edit engine.ini on the server, add:
and make the file read only after you changed it.
To have more slots per role you need to add bots.
Edit game.ini and change:
If you need the appropriate section where the commands belong to, have a look at the server guide. It's documented there
The serverbrowser will still tell you it's a 8player server, but people can connect even if the browser says 8/8 connected.