how to find unmask imei number on icloud for the phone that is not working

The IMEI should be on the box of the iPhone. You can use the serial number instead.

You might also want to read this: iPhone X Display Module Replacement Program for Touch Issues - Apple Support

And you can also log in to https://appleid.apple.com and click on the link below the device to see the IMEI and serial number.

IMEI number should be etched onto the SIM tray too.