CC Madhya 20.320

brahmāra eka-dine haya caudda manvantara
caudda avatāra tāhāṅ karena īśvara
Word for word: 
brahmāra eka-dine — in one day of Brahmā; haya — there are; caudda — fourteen; manu-antara — changes of Manu; caudda — fourteen; avatāra — incarnations; tāhāṅ — in that time; karena — manifests; īśvara — the Supreme Personality of Godhead.
Translation: 
“In one day of Brahmā, there are fourteen changes of the Manus, and during the reign of each of those fourteen Manus, an incarnation is manifested by the Supreme Personality of Godhead.
Purport: 

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.