God is one, we are often told, we may call them by different names but ultimately it’s supposed to be One. This positioning to confirm to the sensibilities of a monotheistic worldview has over the course of past centuries slowly come to take the default position at a superficial level in the minds of most thinking, accommodating and believing individuals, monotheists or otherwise. It may have to do with dominating narratives of monotheistic civilizations but does that make it most sensible? Why, if any, constraints should we apply on the number(s) of Gods out there?
It is not as if limiting the number to One somehow makes the whole thought any more reasonable than say keeping it at 1 million. In fact, if the best form of governance is a democracy, why would you think that the Universe is a dictatorship? If we look deeper, monotheism is in fact a progression towards atheism, for example, Christianity and Islam which came to replace Roman and Arab paganism, did so by denying all other Gods worshiped in those populations and restricting the existence to only one, a logical continuation of this arbitrary denial would be no God at all.
What a lot of practicing Hindus follow in India, barring some who subscribe to the monism of Advaitavad or such, or in many cases even them, is a form of Polycentric Polytheism (It’s funny that some new age spiritualists look down upon Murti Pooja, while the doyen of Advaita – Adi Shankara – himself asked people to build more temples, worship more and wrote beautiful poems for many Gods!).
There are innumerable Gods, but a devotee chooses the one he/she relates to while not denying the existence of other Gods. Which means that a Shaivite for example, would consider Shiva as the Supreme being per his belief, but still won’t deny the existence of Vishnu. He may even bow down to Vishnu but would be chanting Om Namah Shivaay in his head!
This kind of approach is extremely inclusive, because, for a Hindu dharmic, other Gods, even from monotheistic traditions become a part of the pantheon, or are atleast acknowledged as valid, and they do so without making a claim to exclusivity.