The number below the flap indication is not the position of the flaps/slats, but the position of the flap lever. When the lever is moved, the number becomes cyan indicating that the flaps/slats are in transit and when they have reached the selected position the number becomes green.

If the slats or flaps have a mechanical failure the number indication works as mentioned above. If there would be a total brake down of the flaps and slats control computers (we would be talking about quadruple computer failure as both slats and flaps have two controllers) the number would indicate amber XX as there would be no information about the position of the flap lever available to the aircraft systems.

The number below the flap indication is not the position of the flaps/slats, but the position of the flap lever. When the lever is moved, the number becomes cyan indicating that the flaps/slats are in transit and when they have reached the selected position the number becomes green.

Edit: As per FCOM, there is no mention of amber XX indication for flap lever position at all. Checked in a simulator that total computer failure will not cause indication to turn XX.