CC Madhya 20.320
caudda avatāra tāhāṅ karena īśvara
From this verse it can be calculated that in one month (30 days) of Brahmā’s life there are 420 manvantara-avatāras and that in one year (360 days) of his life there are 5,040 manvantara incarnations. Thus for the one hundred years of Brahmā’s life, there is a total of 504,000 manvantara-avatāras. In addition, the Manus themselves are considered partial incarnations of the Supreme Personality of Godhead.