Hi,
It is not possible to identify which device is using the room device license. Cisco allows you to over provision room devices with same feature set. If all of the 9 devices are Online then you will see License Usage: 9/7 with License Overage warning message. If some devices (for example 4) are Offline (but still configured in Webex Control Hub) then you will see License Usage: 5/7 without any warning message.
You need to work with your Cisco Partner or Cisco Account Manager to purchase additional room device licenses otherwise you will receive License Overage bill.
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Thanks, Vaijanath S.