The list of connected devices is in the My devices section. It shows how many slots are taken, which devices are using the subscription and when each of them was last in touch. Anything you no longer need unlinks with the button in its row, and the slot frees up immediately.
Press a row to open the device details.
The device disappears from the list and the slot frees up right away. If you later connect from it again with the subscription link, it reappears in the list and takes a slot once more.
There is nothing to add: a device appears by itself when an app reaches your subscription link. The browser extension lands in this list too.
Reinstalling an app, resetting a device or moving to another phone can create a new entry, and the old one stays until it stops being in touch. Inactive devices are deleted automatically, so you do not have to tidy the list by hand.
Check the obvious first: an old phone, a work laptop, a TV or the browser extension all sign in differently, and a familiar device can look new after an app reinstall. If there is no explanation, unlink the entry, see whether it comes back, and then write to us in the @obfsnbot bot.