What is the lowest combination of dual-types that can resist/be immune to every single possible combination of 2 types?

Question

The lowest amount of dual-types required and what they are to resist each possible combination of 2 different types.

Basically what I'm asking is for the lowest amount of dual-types needed to have one dual-type that resists/is immune to both types of another given dual-type, and have that for every dual-type.
I'm getting confused writing this, but I hope you understand.

Thanks in advance.

Comments

This is asking about existing Pokemon, not type combinations, right? I can't say the answer includes a normal steel Pokemon?

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monkey business is right, and you can say non existing types if you want. its type combinations.

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Then how are we supposed to consider abilities? Pokemon have abilities, type combinations don't.

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Well, you can choose to only use existing types and include abilities, use non-existing types too and don't include abilities, OR just assume that non existing types don't have any abilities. Do it how you want, whichever is easiest to calculate because I'm not picky.

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Is anyone still looking at this question??