If you accept the existence of a deity, then how do you know there is only one?

[gtar100]

44. In an eternal and infinite universe, any God or Gods that one can fathom

can just as equally be encircled by greater beings. Fractal patterns is one way to imagine this sort of thing. It's completely useless to limit a god to any definition until one wants to create a reality. Then laws of nature are created, boundaries are made and possibilities come into existence. Eventually we humans become a thing in this reality and for whatever the reason many of us wonder if the whole thing was dreamed up by some omniscient dictator or by committee. Or did a metaphorical rock start tumbling down a metaphorical hill and in an endless series of accidents and chemical reactions, it all came into being with nobody at the metaphorical wheel.

Evidence I take that there be a god or gods is that there is intelligence in the universe that does not seem so random to me and the model fits well with how I think. Your mileage may vary.