If you’re looking for on/off (no alpha blending), then you can have 256 materials just by using each RGB value for a different material.
If you want to blend them, then you’re out of luck because Cyan is a mix of the Green and Blue channels from RGB, and there’s no way to tell whether you meant to mix the G and B textures, or if you wanted the Cyan texture… And even then, no way to tell what you wanted to blend the Cyan texture with.