The Incarnations of Lord are unlimited. However Supreme Vishnu manifests himself in 3 primary forms viz. Karnodakshayee vishnu(Maha-Vishnu), garbhoDakshayee Vishnu and Ksheerodakshayee Vishnu. Supreme Vishnu first manifests as Sankarshan(1st expansion) and Sankarshan manifests himself as mahavishnu. Mahavishnu lays down in causal ocean(Kaaran ocean;hence called Karnodakshayee Vishnu).From MahaVishnu unlimited universes come into existence.Vishnu enters each of the universes as garbhoDakshayee Vishnu from whose naval brahma is born. the stem that grows from His navel has all planetary creations in universe. Ksheerodakshayee Vishnu then lies in Ksheer saagar(Ocean of milk) and maintains that universe and also enters each and evry entity as paramatma in that universe.
These innumerable universes are produced from the pores of
Mahā-viṣṇu’s body. As innumerable atoms pass through the tiny holes in
a screen, innumerable universes similarly emanate from the pores of
Mahā-viṣṇu’s body. As He breathes out, innumerable universes are
produced, and as He inhales, they are annihilated. All of the energies
of Mahā-viṣṇu are spiritual: they have nothing to do with the material
energy. In the Brahma-saṁhitā (5.48) it is stated that the
predominating deity of each universe, Brahmā, lives only during one
breath of Mahā-viṣṇu. Thus Mahā-viṣṇu is the original Supersoul of all
the universes and the master of all universes as well.
The second Viṣṇu incarnation, Garbhodaka-śāyī Viṣṇu, enters each and
every universe, spreads perspiration from His body and lies down on
that water. From His navel grows the stem of a lotus flower, and on
that lotus flower the first creature, Brahmā, is born. Within the stem
of that lotus flower are the fourteen divisions of planetary systems,
which are created by Brahmā. In the form of Garbhodaka-śāyī Viṣṇu, the
Lord maintains each universe and tends to its needs. Although He is
within each material universe, the influence of the material energy
cannot touch Him. When it is required, this very same Viṣṇu takes the
form of Lord Śiva and annihilates the cosmic creation. The three
secondary incarnations – Brahmā, Viṣṇu and Śiva – are the
predominating deities of the three modes of material nature. The
master of the universe, however, is Garbhodaka-śāyī Viṣṇu, who is
worshiped as the Hiraṇyagarbha Supersoul. The Vedic hymns describe Him
as having thousands of heads.
The third incarnation of Viṣṇu, Kṣīrodaka-śāyī Viṣṇu, is the
incarnation of the mode of goodness. He is also the Supersoul of all
living entities, and He resides on the ocean of milk within the
universe. Thus Caitanya Mahāprabhu described the puruṣa-avatāras.
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Lord vishnu has two expansions. Sva amsha and vibhinna amsa. Lord narayan, Lord ram etc are his sva amsha. ie.they are indifferent from Krishna. Krishna is called Supreme person in Srimad Bhagvatam. gaudiya Vaishnav regard Him as Supreme Personality of godhead or Supreme Brahman.
Lord Brahma,demigods,humans etc are vibhinna amsa.
Isopanishad says:
The Personality of Godhead is perfect and complete, and because He is
completely perfect, all emanations from Him, such as this phenomenal
world, are perfectly equipped as complete wholes. Whatever is produced
of the Complete Whole is also complete in itself. Because He is the
Complete Whole, even though so many complete units emanate from Him,
He remains the complete balance.
Hence all the incarnations are as good as Supreme Vishnu himself. But one should not consider Krishna(Supreme lord) to be on same level as demigods or we;living entities.
Also lord being omnipresent; he is present evrywhere. hence when he descends to this world; it does not mean that he is absent in Vaikuntha. He in evrywhere.
He is the master of maya(illusion). so by his potency he appears in this mortal world as if one of us.
Lord explains this in gita chpt 4(4.9).
P.S: This answer is based on interpretation of Gaudiya vaishnav.
