I came to know that "Indra" is nothing but a designation/post given to a person. so I want to know that how many Indra are there till now and what was there family background (like there real name,parents etc;) and how they became the king of devas(gods).
Indra is a name of the Supreme Reality (Brahman) and it is NOT a post as per the Vedas.
Please remember that the sayings of the Puranas are acceptable so long as they do not contradict the Vedas. In the Vedas, Indra is one name of the Brahman or the Supreme.
In the Kaushitaki Upanishad, there is mention of the Indra-Pratardana-Samvada.When Pratardana asked Indra : What is the highest goal of life?, Indra replied : Know Me. The commentaries of the Brahma-Sutras accept that here by 'Me', Indra means the 'Supreme Brahman'.
The RigVeda-SamhitA(1/164/46) reads
indram mitram varunam agnim ahuh..ekam sad viprA vahudhA vadanti, Meaning: The same Absolute Reality is named as Indra, Mitra, Varuna and Agni.
It also reads along with the Brihadaranyaka Upanishad:
indro mAyAbhih pururupa iyate. Meaning : Indra takes different forms by Maya (Rig veda 6.47.18 and Brhadaranyaka 2.5.19)
The Manu-Samhita also writes
enam eke vadanti agnim manum anye prajApatim/indram eke pare prAnam apare brahma sAsvatam//(12.123), Meaning: The same Eternal Brahman is named by some as Agni, by some Manu, by some PrajApati, by some Indra and by some as PrAna.
The word 'Eternal' is noteworthy in the above line. Surprisingly one name of Lord Vishnu in the Vishnu-Sahasranama is 'Upendra' and 'Upa' means 'Inferior'. So should we understand that Vishnu is inferior to Indra? As per the Vedas no such comparisipon should be allowed, as any part of the 'Purna' (Infinite) is also 'Purna' as per the very Vedas.
I repeat, What the Vedas say is FINAL.
Reference: Smritir Aloye Swamiji, Swami Purnatmananda(Ed.), Udbodhan, page 445-6.M-kar-Baba,Mahamilan Math, page 120-1.