CC Madhya 20.322

śateka vatsara haya ‘jīvana’ brahmāra
pañca-lakṣa cāri-sahasra manvantarāvatāra
Synonyms: 
śateka vatsara haya — there are one hundred years; jīvana — the duration of life; brahmāra — of Brahmā; pañca-lakṣa — 500,000; cāri-sahasra — 4,000; manu-antara-avatāra — incarnations who appear during the reign of each Manu.
Translation: 
“During the hundred years of Brahmā’s life, there are 504,000 manvantara-avatāras.