There are billions of Garbhodakashayi Vishnus and Brahmas but only ONE Karanodakshayi Vishnu (Maha Vishnu).

The entire material creation originates from Karanodakshayi Vishnu's (Maha Vishnu) dreams Srimad Bhagavatam explains. 

Srila Prabhupada - "All existence in the material world is created by the dreaming of Maha-Vishnu. The real, factual platform is the spiritual world, but when the spirit soul wants to imitate the Supreme Personality of Godhead, he is put into this dreamland of material creation”. (Srimad Bhagavatam Canto 4 Chapter 29 Text 83)

Garbhodakashayi Vishnu, seen in painting below, is an expansion of Karanodakshayi Vishnu and resides deep inside each of the billions of gigantic Brahmanda material universes that come from the Body of Maha Vishnu (via His dreaming as He sleeps) as seen in painting below.

Each massive Brahmanda universe surrounds and encases an inner secondary smaller universe where Garbhodakashayi Vishnu resides.

The massive size of our outer Brahmanda that encases our universe, is 44 quadrillion 444 trillion 444 billion miles in diameter according to Srimad Bhagavatam.

Or twice that size if measured from one side of the outer edge of our inner universe that the Brahmanda surrounds.

Also originating from Garbhodakashayi Vishnu is Lord Brahma who eventually creates (builds) the 14 Planetary Systems within our small enclosed material universe inside our Brahmanda.

All the inner material universes that are deep inside each Brahmanda, are of different sizes, our inner universe deep inside our Brahmanda, is only 4 billion miles in diameter and has 14 Planetary Systems in it according to Srimad Bhagavatam's 5th Canto.

Therefore, the inner material universe our Garbhodakashayi Vishnu and Brahma are in, is very small compared to other material universes deep inside their Brahmanda universe.

In this way, there are billions of Garbhodakashayi Vishnus and Brahmas, but only ONE Karanodakshayi Vishnu (Maha Vishnu)

So to be clear, each inner material universe is surrounded by and encased in a massive outer greater universe called a Brahmanda, which is made up of 7 material elements (Other places in Bhagavatam says 8 material qualities)

As said above, all Brahmanda spherical universes originate from Maha Vishnu (Karanodakshayi Vishnu) as seen in paintings below.

The first expansion of Lord Krishna the Supreme Personality of Godhead and cause of ALL causes, is Balarama and from Him Maha Vishnu manifests and creates the material worlds (Brahmanda universes).

From Maha Vishnu's breathing and through the pores of His skin, all the individual massive universes (Brahmandas) manifest.

As explained above, the diameter of our Brahmanda is 44 quadrillion 444 trillion 444 billion miles in diameter, or double that size if measured from the outer edge of our inner secondary  universe from one side.

Also others Brahmandas with a secondary universe inside, are far larger than our one.

So lets be clear, deep inside each Brahmanda universe is found Garbhodakashayi Vishnu, from Him Lord Brahma appears on top of a lotus stem on a lotus flower growing from Garbhodakashayi Vishnu's naval.

And from Brahma comes all the different planetary systems within his individual material universe.

As already said, there are billions of Lord Brahmas each managing their own inner secondary material universe deep within their outer massive Brahmanda universe.

Also Maha Vishnu expands again to a third Vishnu known as Ksirodakashayi Vishnu Who is the supersoul that is within the heart of all material bodily vessels alongside the marginal living entities (jiva tattva souls) possessing those material bodies given by Maha Vishnu.

The material bodily vessels the jiva souls occupy in the material creation, are chosen from Maha Vishnu's dreams because the entire material creation are the dreams of Maha Vishnu according to Srimad Bhagavatam.

Srila Prabhupada – “This material creation is the spirit soul’s dream. Actually all existence in the material world is a dream of Maha-Vishnu, as the Brahma-Samhita describes-

“This material world is created by the dreaming of Maha-Vishnu. The real, factual platform is the spiritual world, but when the spirit soul wants to imitate the Supreme Personality of Godhead, he is put into this dreamland of material creation”. (Srimad Bhagavatam Canto 4.29.83)

This material creation is the spirit soul’s dream. Actually all existence in the "material world" is a dream of Maha-Vishnu

Srila Prabhupada – “Everything happening within time, which consists of past, present and future, is merely a dream. This is the secret in understanding in all the Vedic literature.” (Srimad Bhagavatam Canto 4.29.2b)

Srila Prabhupada – “Our contact with matter is just like dream. Actually we are not fallen. Therefore, because we are not fallen, at any moment we can revive our Krishna consciousness, we break the dream.” (Tokyo Japan 1972: Srimad Bhagavatam 2.9.1)

Srila Prabhupada – “Factually all of material existence is only a dream. Thus there is no question of past, present or future. Persons who are addicted to karma-kanda-vicara, which means ‘working for future happiness through fruitive activities’, are also dreaming. Similarly, past happiness and present happiness are merely dreams” (Srimad Bhagavatam Canto 4.29.2b)

The jiva soul's (anti-material beings) are NOT part of Maha Vishnu's material dream, only material planets and the material bodily vessels are from the dreams of Maha Vishnu.

All material bodies belong to Maha Vishnu that we, jiva souls, get from Maha Vishnu so we can experience our rebellious material desires while trying to enjoy separate from Krishna in the temporary material creation.

This material creation is NOT the jiva soul's real home, our real home is Vaikuntha or Goloka Vrindavan in the perpetual Spiritual World.

Maha Vishna's dreaming creates the material worlds (Brahmandas) that provides material facilities and material bodies to the jiva souls who have chosen to enter the material creation.

No jiva souls originate from Maha Vishnu, they instead go through Him after leaving Vaikuntha Prabhupada has explained to us.

Maha Vishnu then plants the jiva soul in a suitable material body on a suitable planet according to each jiva soul's material desires.

But the jiva souls existence in the material creation is also their dream according to Srimad Bhagavatam, a dream within a dream.