According to the purna avatara of Vishnu, in Mahabharatha, depicted in Bhagavad Gita, In his complete form Vishnu advises yoga states and core of the upanishads and vedas to Arjuna.At the start He shows himself as the "one and only",with all the gods and entire cosmos in him. Being "Advaita". The question is: how come Hinduism still is the most supported Polytheism? Why did Krishna Give the knowledge of vedas to a Kshatriya,as Arjuna was instead of anyone Brahmana kula ? Why should the single god as in Krishna worship someone in himself as in Shiva?
Hinduism recognises one non Dual Supreme God. Many Gods seen are just the forms of the one Supreme Lord. Actually these Gods are called Devatas but in English they are wrongly translated as Gods, some also translate them as demi-gods.
So, most of the Gods in Hinduism are either direct form of Supreme Lord or they are just title and one can get them by doing virtuous action. For eg. Indra is a name of Devata (god), but every person has potential to become Indra by doing Virtuous action.
However the Supreme Lord is non dual and only one.
Svetasvatara Upanishad (6.11) states:
एको देवः सर्वभूतेषु गूढः सर्वव्यापी सर्वभूतान्तरात्मा। कर्माध्यक्षः सर्वभूताधिवासः साक्षी चेता केवलो निर्गुणश्च ।।
The non−dual and resplendent Lord is hidden in all beings. All−pervading, the inmost Self of all creatures, the impeller to actions, abiding in all things, He is the Witness, the Animator and the Absolute, free from gunas.
As the term Eko Deva is used it means one Lord.
But now philosophical difference arises. As per Advaita the self of all including Gods also is Nirguna Brahman which is formless and attributeless. So, Nirguna Brahman became the only one Lord and self of all.
As per Vishishtadvaita one God which has form is the self of the self or Antaryami of the self. As per Vaishnavas it is Lord Vishnu who remains as self of self and as per Shaivites it is Lord Shiva who remains as self of self. But still they all tell there is only one God who is self of self.
Regarding many varieties of Gods having various name and forms, they are just manifestations of the single non dual God.
Regarding worship of different Gods, if they are worshipped by desiring salvation/ liberation all finally lead towards the single non dual lord finally. Skandha Purana [Yajna Vaibhava Khanda] states:
यथा तोय प्रवाहाणां समुद्र परमावधि ।
तथैव सर्व मार्गाणां साक्षान्निष्ठा महेश्वर ।।
Just as all path of streams finally reach to the Great Sea. Similarly all Paths for God lead to the Great God himself.