Books : Srimad-Bhagavatam : Canto 1:"Creation" : SB 1.3: Krsna Is the Source of All Incarnations : SB 1.3.5 : PURPORT :
The next incarnations are the Manus. Within one day's duration of the life ofBrahmā (which is calculated by our solar year as 4,300,000 x 1,000 years) there are fourteen Manus. Therefore there are 420 Manus in one month ofBrahmā and 5,040 Manus in one year ofBrahmā. Brahmā lives for one hundred years of his age, and therefore there are 5,040 x 100 or 504,000 Manus in the duration ofBrahmā's life. There are innumerable universes, with one Brahmā in each of them, and all of them are created and annihilated during the breathing time of the puruṣa. Therefore one can simply imagine how many millions of Manus there are during one breath of the puruṣa.