1. Let the definition of god (for the sake of this thread) be "an omnipotent, onmipresent, and omniscient being".
2. If multiple gods exist, they can interfere with each other.
3. If one being can interfere with another, then that other being is not onmipotent. For one, if both beings are truly omnipresent - i.e., present throughout all time, and through eternity - then one cannot destroy the other. They necessarily limit each other's abilities.
4. Therefore, if you believe in many gods, you don't believe in gods, but rather something akin to superheroes or other mythical beings.
Let me note that I have no stake in the matter, I'm atheist anyway - but I never understood how people can square belief in many "gods" with the idea that those beings are actually "gods".