Please note that the counter in the top left corner does not reflect the total limit of your account. Please note that the blue number here (40) stands for the push-enabled devices count. The white number (devices total, 58) reflects the overall amount of devices ever subscribed for notifications, including those devices that were unregistered (have no valid push token) to the present moment.
As for inactive users (those who don't have a valid push token and did not use the app for 90 days) - we clear them out of the system to keep the user base as up-to-date as possible. Please also note that should any of such users open the app again and agree to receive notifications they will once again be among the active subscribers.
As for the account limit, you can see it in the "My Account" menu of the Control Panel.