Yes this can be controlled in a couple of ways.
Amigopod operator profiles allow you to control the number of 'guest accounts' a sponsor can create as of 3.5 software. When you edit an operator profile there is a set of new filters around how many accounts can be created underneath all of the access control options. So you can set up an operator profile to let the user provision their own devices via self registration or by creating a user/mac device
The second thing that can be done is limit the number of sessions a single username/password can have concurrently. This may not fully support your use case but is an option. This can be set universally under Customization/Guest Manager or you can add a radius attribute (max sessions) to restrict on a per role basis
Hope this helps
Carlos