Why does Hinduism have so many Gods?

Anyone who is new to Hinduism would have heard somewhere about the millions of Gods the religion has. It is very unfortunate that there are many naïve Hindus who do believe the multitude of Gods. The simple truth is that Hinduism specifies only one God. If that's so, what about the other millions? Let's get it clarified.

Devas - The Divine Beings:

In nature, we see wild beasts that are much mightier than human beings, outmatching the latter by sheer physical strength. In contrast, human beings are intellectually far superior to these beasts. Hinduism specifies a few others. One kind, called the Devas are divine, heavenly beings far superior to human species in both physical might as well as intellect. Similar to the different races we have, Devas are further classified into many sects, like Kinnara, Kimpurusha, Yaksha, Siddha, Sarana, Sadhya, Gandharva, etc. They are responsible for maintaining balance among the natural elements that help in the sustenance of life and creation.

Another popular classification of the Devas include 8 Vasus, 11 Rudras, 12 Adityas, and the Asvini twins, making them 33 in total [*]. Each of them has a crore (10 million) of subordinate Devas totaling their population to 33 crores. This has notoriously given Hinduism its alleged polytheistic status.

Humans and Devas have a relationship through a kind of trade. Humans need a balance of their natural environment, as in timely rain, or wish for a material need, such as health, wealth and power. On the other side, Devas by their nature depend on the sacrifices from the rituals of humans to survive, making the two interdependent. Hence, almost all of the rituals are intended to please one or more Devas for material needs through the sacrificial offerings. 

Devas and their arch nemeses Asuras churning the ocean of milk to obtain the elixir of life.

Devas have three forms. First, their divine body in the heavens called Adi Daiveeka Roopa. This form can only be perceived or interacted by humans through great Tapas (Yogic methods). Second is called Adi Boudeeka Roopa, which can be perceived in nature through our very senses. For instance, the air around us is a Deva called Vayu. Similarly, water, earth, fire, etc. are specific Devas. The third form is part of our body, called Aadyaatmika Roopa.  Surya (Sun) is in the eye, Agni (Fire) is in the mouth, Indra is in our hands, and so on. 

One may think these elements such as air, water, and fire are just inert things, and that it is irrational to consider them as divinely beings. With a little bit of understanding one can deduce that the world we are in is just one living being with all its entities connected, similar to the millions of living cells that constitute our body. It is as if thinking the blood is just a non-living fluid. Thus, it is not unreasonable to say that we are only a cell or very well just a part of it in this macrocosm. 

Although Devas are divine beings much superior to humans, they are not Gods. They do go through the pleasures and pains that are common among the other living beings.

Multiple Deities for worship:

Hinduism specifies only one God. However, this God can be perceived by humans through various forms as mentioned in the scriptures. When engaged in a specific role, God takes up a certain form. Similar to how a person takes up multiple roles, such as an engineer by profession takes up a role as a parent when caring for their child, a cook when preparing food, a sportsperson when playing, and so on,  so does God who takes up many roles such as Creation, Sustenance and Destruction. 

Lord Shiva as Nataraja (literally, Lord of Dance) in His cosmic dance. The posture symbolizes the five great attributes, viz., Creation (drum - top right hand), Sustenance (dance), Destruction (fire - top left hand), Concealing (Veiling mudra - bottom left hand), and Protection (the Abhaya mudra - bottom right hand).

In addition to these, God takes up forms that would please individuals according to their natural interests, for us to show our love in the form of Bhakti. Love can be shown towards God in several ways, as love towards our Mother, love towards a Child, loving a Friend, a King, a Hero, a Teacher, and even a Lover, again as mentioned in the scriptures.  Thus, we have multiple forms of the same God. Adi Sankaracharya has channelized these forms into six, called Shanmata: Shiva, Vishnu, Shakti, Ganesha, Surya and Skanda [*].

Lord Vishnu - Purushottama (meaning the Absolute Supreme Being) is the divine manifestation of existence; Paramatma, The Ultimate Soul.

Undoubtedly, it is the very same God that is known through various forms. Whichever is the way that is comfortable to an individual, that form is worshiped as directed by the scriptures. 

Reference:

Deivathin kural (Voice of God) Volume 1, Compiled by R. Ganapathy, Vanathi Pathippakam