Where can I find the PSN number on my Nokia or HMD tablet?
Devices with SIM card slots have IMEI numbers. If your device does not have a SIM card slot, like the Wifi-only variant of the Nokia T20, then the PSN can be used to identify your device.
To find your device's Product Serial Number (PSN), head to Settings > About tablet > Model and hardware. There you can find the Product Serial Number of the device.