How to remove a device from my adobe id counter?

https://account.adobe.com/security 

At the bottom page find "Active sessions" and option to "end session" for any/all devices actively signed on.

Log into your Adobe account. Go to your Plans. Click Manage Accounts. Scroll down to Activated Devices.

This will list your devices. If you would like to deactivate a device, simply click Deactivate next to that device.

Into your Adobe ID account on adobe.com, go to Plans & Products and click "Manage Plan."

The right hand column has some items. First is "Payment Details", then "Activated Devices.

Select any activated device in the list and click on the circled X, that will deactivate the device.

If you still have the old devices/computers you can deregister them.

For a computer running ADE, ctrl-shift-D (cmd-shift-D on Mac)

For an ereader device, connect it to a computer running ADE, then ctrl-shift-E (cmd-shift-E on Mac)

For a tablet, find instructions for the app you are running.

Unfortunately, deregistering does not always free up the slot to allow for more devices as is should.

In that case ...

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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=using-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.