If you still have a device registered you no longer need, connect it to ADE and unregister it with ctrl-shift-E.
That should free up its registration and allow you to register an alternative device.
If you no longer have unneeded devices available (passed on, lost, broken) you will have to contact Adobe service,
and ask them to unregister all devices. You will then have to reregister those you still want (including PCs).
Live chat is probably the easiest: http://www.adobe.com/support/chat/ivrchat.html
Unfortunately, Adobe do not provide a proper web interface for viewing/managing registration.
They seem happy to get money from suppliers to provide Adobe DRM, but not to give appropriate matching customer support.
If you want to exceed the max number of registrations again ask Adobe service.
I'm not sure whether they will be able to help or not.