the 18 Major Puranas [Histories]
-- After "Compiling" 18 Puranas, Vyasadeva [ref: Matsya Purana] composed the Mahabharata consisting of 100,000 Verses (SB 12.13.9).
-- Ramayana is an account originally related by Brahma, in 1 Billion Verses ~ Later Narada summarized it to Valmiki, who further presented it to Humans for Artha, Kama, and, Dharma. (12.13.9)
--Therefore, the total quantity of Verses in all the above Puranas/Itihasas (Histories) = 525,000 Verses (12.13.9)
--The Supreme Personality of Godhead revealed the Srimad-Bhagavatam in full to Brahma while sitting on the MahaVishnu's Lotus, before creation. (12.13.10)