If God exists then He must be the greatest of all beings. Indeed, God must be infinitely great. If God was not the greatest then He would not be God. He would be a mere mortal. Now, there can only be one such being because if there were another then there would be something equal to God. And if there were something equal to God then God would no longer be the greatest and if God was no longer the greatest then God would no longer be God. For God to be God He must surpass all others. And to surpass all others He must be the supreme. There can only be one ultimate supreme because if there were two then the other would no longer be supreme. He would no longer surpass all others and if He did not surpass all others He would no longer be God.

So, if there is a God there can only be one.