My answer wasn't about religions, there can only be one God regardless of the system of worship around God. God is God is God; Allah, YHWH, Zeus whatever, there can be only one.
As for polytheism, most polytheistic religions recognise one supreme, but that wasn't the op's point anyway.

There can only be one God, by definition. There's many religions but there can't more than one supreme being. The question of which one is flawed, it makes a false assumption.