You have a maximum of 6 devices you can use at a time on a given ID.
1) If you want more than 6 active at a time, tough.
2) If you have access to a running device you no longer need, deactivate it and that should free up a slot to allow you to activate a new device.
3) If you have a device that is activated but which you have lost, past on or is broken etc so you can't deactivate it ...
Adobe Live Chat: http://www.adobe.com/support/chat/ivrchat.html,
or as a slight short cut try http://helpx.adobe.com/contact.html?product=digital-editions&topic=usi ng-my-product-or-service
Click on 'I still need help' and then you should see 'Chat with an Agent' at the bottom of the page.
'Ask our experts' will indeed just lead you back to this forum.
Sometimes you will get ‘Sorry! All agents are busy— please check back soon.’
Don’t refresh the page, just hang on and it should eventually go to ‘Chat Now, and agent is available’.
They can reset your authorizations, and then you must reauthorize any devices you still need.
(Unfortunately, Adobe haven’t got round to an admin website for viewing and editing authorizations.)
Some of the representatives haven't been properly trained and don't know what to do (and claim there is nothing they can do);
in that case the only way seems to be to give up that chat and try another session hoping for a properly trained representative.
If your problem is with another device using Overdrive, Bluefire, Aldiko or similar third party app, it is recommended not to mention that app when on the chat, just mention that you have run out of authorizations (E_ACT_TOO_MANY_ACTIVATIONS) . Thanks to AJP_Bear for that tip.