In order to hit all monotypes for SE damage, it requires six moves (Ground, Fighting, Ice, Flying, Electric OR Grass, Dark OR Ghost). But you'll still not get SEs on some dualtypes, even those that actually have weaknesses (such as a Bug/Steel, only weak to Fire). And Abilities can cancel out weaknesses (such as Levitate).
Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere