de-authorizing does not cause the number of allowed authorizations to increase
I didn't know that; it makes the Adobe infrastructure even worse than I thought.
I'm not sure what happens when you have to reauthorize the same device; for example after installing a new operating system; or running more than one eReader app on a tablet.
If you contact Adobe they will remove authorizations from all your devices and you can then reauthorize the ones you still want to use.
(I think they'll only do this once you actually run out of authorizations.)
Live Chat is easiest http://www.adobe.com/support/chat/ivrchat.html
Of course it would be better if Adobe supported their DRM infrastructure properly and provided a web page to view and manage authorizations; but don't hold your breath.