If you have already configured your installation via the Home + Control App, the number is available in the application settings: select ;Manage my home ;,select the room in which the thermostat is installed and select the thermostat. You will find there the serial number.
Otherwise, follow these steps to recover the serial number of your thermostat:
1) Power off you thermostat directly in your electrical panel.
2) Take a screwdriver and insert in the hole on the bottom side of the thermostat. Gently press to remove the front panel of your thermostat.
3) On the back of it, you will find the serial number starting with 00:03:50