While any attempt to prove the non-existence of the gods is frequently eluded by changing the definition of what constitutes a "god", the existence of a single god is easier to disprove.
For empirical reasons the entire concept of a single god is rather counterintuitive. There is no example of anything, of which only exists one. So it is rather odd that anybody could even come up with such a concept that contradicts everything that can be observed in the world.
However there is also an argument from logical deduction that sufficiently proves the theoretical impossibility of the monotheist main doctrine:
If certain preconditions allow the existence of one thing, they would also allow the existence of others of its kind. Since existence of two or more things can be totally independent from each other, the existence of one thing is not necessarily able to affect or prevent the existence of another.
If something (e.g. the existence of a god) is possible once, it is also possible twice. The only alternative would be that it is not possible at all. It is either possible or not. There is no in-between.
Infinity is incompatible with singularity, and there is no reason thinkable that would limit the possibility of independent existences (i.e. separate worlds). The concept of infinite possibilities necessarily requires that everything, which is possible, exists in an infinite number.
In short: If there is one, there must be more of them.
Therefore depending on the definition of a god (Some define it as nature itself, others deify humans as gods. The existence of neither can be denied.), either polytheism or atheism are valid options. Monotheism is no rational option at all.