There is only one God in Hinduism we call him Parmatma or Parabramha.
But we believe a small portion of him resides in every living being as soul, so we worship whom we consider as the best in each life form, they are called as Devi and Devata (DemiGods and Demi Goddesses). (Various Avatars, Ram , Sita, Hanuman,)
We also worship mother nature and her various powers, (Varuna - water, Agni - Fire, Sun - Surya, Rain - Indra). We also worship the qualities which make us better. Knowledge is worshipped as Saraswati, Prosperity as Lakshmi and Valor as Parvati and their consorts who bless humanity with these qualities Brahma (knowledge to create life), Vishnu (prosperity to sustain life) Shiva (Valor to destroy life that has fulfilled it's purpose).
In the Vedic period there were 333 million gods - that’s when the population of the earth was 333 million, we now have more than 7 billion gods as we have 7 billion world population!
Now coming to why Rigveda talks about the 33 Gods (11 in heaven, 11 on earth and 11 on mid air) may be it is talking about 33 million
not 33 as these are old text written in Vedic Sanskrit or may be it has some other meaning. But the core principle is the same we have one god or one supreme power and a human being can have multiple ways to reach him/her and can create own version of that supreme god.