Note that there is no "maximum resists an armor piece can have before it is considered an imbuing slot".
It's a Yes/No situation not a property value threshold.
If ANY of the points in a resist are from runic crafting, imbuing or reforging, it's a property slot.
If the resists are ONLY from base resists, exceptional/arms lore bonuses, and material, it doesn't take a slot.
Theoretically, as a result, you can have a resist over 20 that doesn't count toward slots (by having over half the Exceptional bonus fall into one resist, then enhancing), but that's extremely rare.
Similarly, it's possible to have 1 point over item minimum in a resist and it count as a property (though these days, that's usually the result of lowering a resist to minimum with imbuing, to make room for imbuing other properties).