You can manage your License Pool on the Xibo website, by logging in and clicking on the below link:
Next select the License Pool for the type of Player you are using (Android or webOS) to view the Licenses that are already in use. You can delete the License entries for the Displays that no longer need them by clicking the red button with a small bin icon on it. Please note that unless you uninstall the Player on the device using the License, it will try to contact the License server again.
You mentioned that you are using mini PCs for your Xibo Players, suggesting you are using Xibo for Windows? If so, you do not need to use a license with that version of the Player.