SB 8.13.36

O King! I have now described to you the fourteen Manvantaras which appear in the past, present and future. The total duration of time ruled by these Manus is one day of Brahmā consisting of a thousand yuga cycles. The fourteen Manvantaras end (paryayaḥ) after a thousand yuga cycles. Thus ends the commentary on the Thirteenth Chapter of the Eighth Canto of the Bhāgavatam for the pleasure of the devotees, in accordance with the previous ācāryas. Chapter Fourteen Six Functions in the Manvantara 8.14: The System of Universal Management 14. The System of Universal Management 8.14 Summary This chapter describes the duties allotted to Manu by the Supreme Personality of Godhead. All the Manus, as well as their sons, the sages, the demigods and the Indras, act under the orders of various incarnations of the Supreme Personality of Godhead. At the end of every catur-yuga, consisting of Satya-yuga, Dvāpara-yuga, Tretā-yuga and Kali-yuga, the sages, acting under the orders of the Supreme Personality of Godhead, distribute the Vedic knowledge and thus reinstate eternal religious principles. Manu's duty is to reestablish the system of religion. Manu's sons execute Manu's orders, and thus the entire universe is maintained by Manu and his descendants. The Indras are various rulers of the heavenly planets. Assisted by the demigods, they rule the three worlds. The Supreme Personality of Godhead also appears as incarnations in different yugas. He appears as Sanaka, Sanātana, Yājṣavalkya, Dattātreya and others, and thus He gives instructions in spiritual knowledge, prescribed duties, principles of mystic yoga, and so on. As Marīci and others, He creates progeny; as the king, He punishes the miscreants; and in the form of time, He annihilates the creation. One may argue, "If the all-powerful Supreme Personality of Godhead can do anything simply by His will, why has He arranged for so many personalities to manage?" How and why He does this cannot be understood by those who are under the clutches of māyā.