I am not sure I follow; each device should only be able to make a single Azure AD Join (you cannot join twice). However you can be registered against multiple accounts, but only one join. Then there may be a separate limit: total number of devices allowed per user license (e.g. 5 devices), and you could be running into that if the cloud still sees devices that are no longer present. You could clean up stale devices in the portal this way: -us/azure/active-directory/devices/manage-stale-devices