Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 2
मन्वन्तरे हरेर्जन्म कर्माणि च महीयस: ।गृणन्ति कवयो ब्रह्मंस्तानि नो वद शृण्वताम् ॥ २ ॥
manvantare harer janmakarmāṇi ca mahīyasaḥgṛṇanti kavayo brahmaṁstāni no vada śṛṇvatām
O learned brāhmaṇa, Śukadeva Gosvāmī, the great learned persons who are completely intelligent describe the activities and appearance of the Supreme Personality of Godhead during the various manvantaras. We are very eager to hear about these narrations. Kindly describe them.
The Supreme Personality of Godhead has different varieties of incarnations, including the guṇa-avatāras, manvantara-avatāras, līlā-avatāras and yuga-avatāras, all of which are described in the śāstras. Without reference to the śāstras there can be no question of accepting anyone as an incarnation of the Supreme Personality of Godhead. Therefore, as especially mentioned here, gṛṇanti kavayaḥ: the descriptions of various incarnations are accepted by great learned scholars with perfect intelligence. At the present time, especially in India, so many rascals are claiming to be incarnations, and people are being misled. Therefore, the identity of an incarnation should be confirmed by the descriptions of the śāstras and by wonderful activities. As described in this verse by the word mahīyasaḥ, the activities of an incarnation are not ordinary magic or jugglery, but are wonderful activities. Thus any incarnation of the Supreme Personality of Godhead must be supported by the statements of the śāstra and must actually perform wonderful activities. Parīkṣit Mahārāja was eager to hear about the Manus of different ages. There are fourteen Manus during a day of Brahmā, and the age of each Manu lasts for seventy-one yugas. Thus there are thousands of Manus during the life of Brahmā.