What size o-ring is correct for a -12 AN plug?

#212 Or 7/8"x1-1/8" 1/8 thick

Standard 200 series O-ring sizes will work in a jam but the o-rings for AN O-ring boss fittings are actually called out by SAE 900 series numbers which are a little different sizes than the 200 series. For a #12 O-ring boss fitting the SAE number would be 912 and that has a .924" ID with a cross sectional width of .116".