Shrimad Bhāgavata says that the original form of Mahā Viśnu is Nārāyanā which is the imperishable seed of various avatars and to which they all return (I.3.5). Shrimad Bhāgavata refers to 21 avatars of Narayana and the twenty second avatar Kalki is yet to come. Generally, only ten avatars of Mahā Viśnu are often referred to. They are popularly known as dasa-avatars (dasa means ten). They are Matsya, Kūrma, Varāha, Narasimha, Vāmana, Varasurāma, Rāma, Kṛśna, Buddha and Kalki. Out of this ten, Kalki avatar is yet to happen, probably towards the end of kaliyug. Some texts include Balarama in the place of Buddha.
Following are the 22 avatars mentioned in Bhāgavata. They are Sanaka Kumarās, Vāraha, Sage Nāradā, Nara and Nārāyanā, Sage Kapila, Sage Dattatreya, Aakūti, Riśabadeva, Prithu, Matsya, Kurma, Dhanvantari, Mohini, Narasimha, Vāmanā, Paraśuramā, Sage Vyāsā, Rāmā, Balarāmā, Kriśna, Buddha and kalki (I.3.6-25).
Sanaka Kumaras (Sanaka, Sanadana, Sanatana and Sanat kumara) are four young and yet very powerful sages. They went to the abode of Vishnu. They were prevented by the two gate keepers (dwara balaka). The young sages cursed the gate keepers for having prevented them to have darshan of Vishnu. Knowing this Vishnu came out and pacified the sages. Because of the curse of Sanat Kumaras, the door keepers of Vaikuntha were born as demons by name Hiranyakasipu and Hiranyaksha. There is a reference to Sanat Kumara in Chandogya Upanishad. Chapter 7 of this Upanishad begins by saying that sage Narada went to Sanat Kumara and said “Bhagavaḥ adhīhi iti” which means ‘Sir, please teach me.’ Sanat Kumaras are said to be the sons of Lord Brahma and attained siddhi (self realisation) at their young age. Narayana Kavacha (verse 17) prays “Sanath kumaro aavathu kama devath” which means ‘let sage Sanat Kumara protect me from lust.’ (Narayana Kavacha forms part of Srimad Bhagavata (VI.8.). Sanat Kumaras are considered to be the source of knowledge. They are said to be incarnation of Vishnu. This is considered as the first incarnation of Vishnu from His original form of Purusha (Para-atma or Paramatma or the Brahman).
The curse of Sanat Kumaras made the gate keepers born as demons. Out of the two demons, Hiranyaksha submerged the planet Earth in water. Vishnu incarnated in the form of a tiny boar by coming out of Brahma’s nose and He grew huge in size, killed the demon and saved the planet earth. This is known as Varaha avatar, the second incarnation. He is said to give protection while travelling.
His third incarnation is Sage Narada (Bhagavata I.3.7). Narayana Kavacha (Bharavata (VI.8.17) says that Narada protects against drawbacks in the worship of deities. There are 32 transgressions in worshipping a deity. Sins arising out of such transgressions do not accrue with the blessings of Narada.
Fourth incarnation of Vishnu is Nara and Narayana. They are twin brothers born to Dharma Deva and his wife Murti (Narayaneeyam 16.2). Both of them killed a demon after performing tapas for many years. They taught spirituality and won over Cupid, the God of love.
His fifth incarnation is sage Kapila. He was born to Prajapati Kardama and Devahuti, the daughter of Manu Swayambhuva (there are 14 Manvantras. Manvantra is a period consisting of several thousands of years and each such manvantra is ruled by different Manus.) (Brahmanda Purana- Chapters 43 and 50). Vishnu incarnated in the form of sage Kapila as their tenth son (Narayaneeyam: canto 14). Sage Kapila taught about Atma and Self-realization. He is said to be the Lord of Prakriti consisting of three gunas and saves one from bondage of actions (karma).
His sixth incarnation is Dattatreya. He is the master of yoga and guards against abandoning the practice of yoga. He is considered Guru of gurus. There are different opinions regarding this incarnation. Some texts say that He is the incarnation of Brahma some others say He is Vishnu and yet others say Shiva. There is an Upanishad by name ‘Dattatreya Upanishad’. When rishis approached Vishnu for initiation, Vishnu advised them to recite “I am Datta”. There are several Dattatreya mantras. He is the son of sage Atri. The dialogue between sage Bhargava and Datta is known as ‘Tripura Rahasya’, the glory of Goddess Tripura.