How can you change the max clients in Ventrilo using RCon or the server itself?

You cannot change maxslots by rcon, This can only be done in the configuration files.

The free version only allows for 8 max slots. To enable more slots you would need to rent a server from a professional licensed hoster approved by Ventrilo.

Two of the better known professional hosters are http://www.defconservers.com and http://www.planetventrilo.com

For a full list of authorized hosters see: http://www.ventrilo.com/hosting.php

Companies not on the list are not authorized to sell ventrilo products.