I don't think the number of licences can be increased.
The whole Adobe DRM system is an amateur mess; there doesn't seem to be any way to see what devices are authorised, or to remove authorization for individual devices.
It appears from the FAQ ( http://www.adobe.com/uk/products/digital-editions/faq.html , 'What is the maximum number of computers and devices that I can activate?' )
that the only thing you can do is to get Customer Services to reset everything, and then reregister the devices you still want to use.
You need to follow two links from the FAQ ( http://helpx.adobe.com/digital-editions.html to http://www.adobe.com/support/digitaleditions/supportinfo/ )
which comes up with an online chat. I haven't needed it yet, but I assume they can do the necessary reset.
That said, I had to do quite a lot of re-registrations because of bugs in ADE 2.0's handling of Sony PRS-350,
which involved reinstalls and reregistrations of ADE on different machines and several times on the Sony.
I'm sure I've gone over 6 total and not yet had any complaint from the ADE software;
so maybe it is sensible enough to realize when a registering device is the same as one that is already registered and not up the count.