The Mahapuranas contain 400,000 slokas . They are four times the size of Mahabharata . In these, six are Satvic or Vaishnava Puranas and glorify Lord Vishnu, six are Rajasic or Brahma Puranas and glorify Lord Brahma, and six are Tamasic or Shiva Puranas and glorify Lord Shiva.
In Puranas, the supreme truth is made known to one and all, including ordinary men, in a very simple manner....There are many Puranas. It is believed that there were approximately 64 Puranas consisting of 18 Mahapuranas, 18 primary Upa Puranas and the rest secondary Upa Puranas.
The 18 Mahapuranas (great Puranas), as the origins of the Puranas, may have overlapped to some extent with the Vedas , but their composition stretched forward into the 4th-5th centuries,... the earliest parts of the Puranic genealogies are either entirely or partly w:Mythicalmythical. The oldest of the Puranas are the Matsya, Vayu and the Brahmanda and for our purposes, the Vishnu Purana, somewhat later than the first three ... the Vedic link also goes back to the earlier statement that the itihasa-purana was the fifth Veda.