Is hinduism a polytheist religion?
Hinduism cannot be characterized in this way. Because 'personal belief' (we term it as 'mata' - opinion) is unrestricted, free. So, personal beliefs differ from one hindu to another. There is no religious 'ism' (I checked all in Wikipedia), which is not available in hinduism. There are polytheists (my family is one), pantheists, pansntheists, henotheists, kenotheists, transtheists, monotheists, and atheists (yours faithfully is an 'advaitist' (non-duality) atheist hindu). But there are no apatheists or agnostics, it is either yes (for majority) or no (for a minority).
What is common in all hinduism (with minor variations) is 'dharma' (fulfillment of one's familial and social duties and engaging in righteous action - helping others and not being the cause of pain to others). The variations could be about marriage (patriarchial or matriarchial, marrying second cousins or not, etc.) or food or traditions (when should the year start - India has some 30 well-developed religious calendars) particular to family or caste.
So, yes, hinduism is polytheist and no, not all hindus are polytheists. With best regards,