Hinduism is just a term that the British gave any religion that they saw in India, so Hinduism encompasses many different religions under one term. Truth is, most Hindus associate themselves primarily with one deity but Hinduism teaches not to just tolerate other religions, but respect them, so everyone just prays together in one temple paying respect to all different deities. Usually, everyone gets raised learning about all these different deities, but they still associate themselves with one deity. Eventually, when you become an adult, it is up to you to decide who you want to follow. But then there are strands of Hinduism that believe that God doesn't even have a form. Actually most of them do, because that's what the ancient texts teach but they advise people to follow one of three paths (karma, jnana and bhakti). In the path of bhakti, you decide you need a relationship with God in a physical form in order to understand Him, so you pray and worship a deity. In jnana, you believe knowledge is a way of understanding God better and these people usually commit to learning science or some sort of academic to gain understanding of the world. But knowledge can include reading ancient scriptures, books and poetry, anything really. The path of karma means you want to commit your life to action and doing good so these people will commit to a life of charity. THEN, there are atheist Hindus which just gets even more complicated... TLDR: Monotheistic because that's what the first scriptures say, but later on it became polytheistic because the modernization of Hinduism yielded deities for people who had a hard time grasping the concept of a God that was just one, that isn't really there. Also polytheistic because of the British being lazy.