Hinduism is a religion that defies definite classification, and in practice, it has both monotheistic and polytheistic components. With over a billion adherents, many claim that the religion is monotheistic and helmed by the supreme being, Brahma. However, by the definition of polytheism, Hindu appears polytheistic because there are legions of Gods and Goddesses that describe aspects of Brahma's nature.
Major proponents of the monotheistic nature of Hinduism point to the fundamental trinity of Brahma, Shiva and Vishnu as three aspects of a single entity, similar to the Christian trinity of God, Jesus and the Holy Ghost. Brahma is the creator aspect of the supreme being, Shiva is the destroyer and Vishnu is the preserver.
From there, however, it gets more complicated, as many Hindus see the various aspects of Brahma manifest in hundreds (potentially thousands) of other deities. The actual numbers are almost unknowable, as there are those, like Ganesh, whose following is more widespread, to local deities whose names may only be known to a small few who live in that particular area.
In the end, the question of "monotheistic or polytheistic" is somewhat unimportant. What makes Hindusim different from other religions is that it allows for both to be true, with neither choice being in conflict with the other.