As long as they access the same database you can control multiple sites with a single powercms back end.
The license is for installation of the admin, so it can run any number of sub domains as long ast they all access the same content database.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.