Take this for example.
You respect your uncle and aunt, as well as a few of your far relatives, just as much as your parents. Does that mean these people are really your parents?
Similarly, Hindus worship many idols. But understand that these idols aren’t really "God". They are Demi-god or minor deities. They are foremost praying to one single God.
Yes, in reality, in Hinduism, there's only one God: Brahman or a Supreme Being. And then there are 33 million demi-gods or devas and devis.
All the demi-gods are the manifestation of Brahman, in different forms and figures. They are called and revered differently by different sects and communities within Hinduism.
The three main forms of Brahman are:
• Brahma: created the universe
: preserves the universe
: destroys the universe
Now each of these forms of Brahman has its own several avatars. Like, Ram, Krishna, Durga, Parvati, Narasimha, and more.
So, while Hindus are usually considered to be polytheists, at the core, they are monotheists. Through different names, avatars and manifestation, they are basically worshiping the same Supreme being. Sadly, this belief isn't widely prevalent… even among the Hindu communities, who sometimes absurdly battle out Gods. "Who will win between Krishna and Rama?" In reality, both are the same!
To conclude, to answer your question, Hindus, in the end, are worshiping one God.
However, they are just revering different avatars of the same God. Why? Because different avatars of Brahman are known to have their distinct task or power. For instance, when people want rain, they worship Indira. For fertility, Shiva is worshiped. For the money, Laxmi is worshiped. Each of these God's manifestations or avatars are known to have their respective roles.
Hope this answers your question!
(Courtesy: Dolls of India)