Am told by city inspector that the distance is 8". But the origional installation (20 yrs ago) was 12". Who is correct?

Depends on what city you live. And what Country. There is no one universal international code everyone follows.

Unless he can provide some sort of documentation for this claim there is no set distance.

Most electric range receptacles are right at the floor, because that is where the void is behind the range.

If you are in the US, then the installation instructions have more authority than the inspector... NEC 110.3(B)

If Article 250.50(C) - "Appliance Receptacle Outlets" is applicable, then the receptacle is required to be within 6 ft. of the appliance.