Back when mainstream American comic books began, early in the 20th century, there was only one narrative and only one fictional universe. This lasted for some decades, but at one point, publishers began feeding us with so-called alternative stories, which gave birth to the concept of the Multiverse.
The Multiverse is a concept of a larger fictional universe within which there exist multiple alternative realities, often called “Earth”, with one primary, canon narrative setting; in Marvel’s case, it is Earth-616. This allowed for more diverse storytelling and enabled a constant expansion of the fictional narratives.
The concept of the Multiverse might seem difficult to understand, but that is quite misleading. We are going to explain everything step-by-step and before you’re done, you’re going to be an expert in the field. We are also going to explain how the MCU universe ties into the whole concept.
What is Marvel’s Multiverse?
Within the narrative universe created by Marvel Comics, most tales take place within the large fictional Marvel Universe, which, in turn, is part of a larger fictional Multiverse. Beginning with the early issues of Captain Britain, the main continuity (the so-called “Prime Earth”) in which most of the Marvel stories take place was labeled as Earth-616, and thus, the Multiverse was established as being protected by Merlyn.
This was back in 1976, although the concept was vaguely introduced as early as The Avengers #85 (February 1971). 
The concept worked so that each universe has an iteration of Captain Britain appointed to protect their iteration of the British Isles. These protectors are collectively known as the Captain Britain Corps and they actually represent the initial evolution stage of the Multiverse.
Gaining popularity, this numerical notation of the individual Earths was continued in the Excalibur series and other titles before pouring over to the core titles.
Each alternative universe within the Multiverse in Marvel also had its own Sorcerer Supreme at almost every moment in their history, always appointed by the mystical trinity of Vishanti in order to defend the world against threats of primarily magical nature from within and beyond and always wielding the Eye of Agamotto, the Sorcerer Supreme’s most powerful weapon.
Later, many writers would use and reshape the Multiverse in different titles, with some notable being Exiles, X-Men, and the Ultimate Fantastic Four; the Ultimate Marvel Universe is especially noteworthy in this context.
New universes would also emerge from stories involving time-traveling characters like Rachel Summers, Cable, and Bishop, as their actions made their origin times alternate timelines.
How many universes are there within Marvel?
The Marvel Multiverse opens the possibility for an infinite number of alternative Earths, as it is not, unlike DC’s Multiverse, limited in any way. The exact number of alternative Earths is unknown, but here is a list of some of the more prominent ones:
|Earth-616||Motion Picture Funnies Weekly #1 (1939)||The main setting of the Marvel Universe.|
|Earth-1610||Ultimate Spider-Man #1 (2000)||The Ultimate Marvel universe is the reinvention of the Marvel Universe for the modern age, Initially beginning with Ultimate Spider-Man and Ultimate X-Men.|
|Earth-199999||Iron Man (May 2, 2008)||The Marvel Cinematic Universe (MCU) continuity.|
|Earth-10005||X-Men (July 12, 2000)||Original setting of the X-Men film franchise until the events of X-Men: Days of Future Past.|
|Earth-92131||Night of the Sentinels (Part 1) (Oct. 31, 1992)||Setting of X-Men: The Animated Series, Spider-Man: The Animated Series and (possibly) other animated shows of the early 1990s.|
|Earth-12041||Great Power and Great Responsibility (April 1, 2012)||Universe of the animated series featured on Disney XD.|
|Earth-17628||Marvel Universe Guardians of the Galaxy #1(February 2015)||Reality of Marvel’s Guardians of the Galaxy, Marvel’s Spider-Man, and Avengers: Black Panther’s Quest (Season 5 of Avengers Assemble).|
|Earth-8096||Wolverine & the X-Men (January 23, 2009)||Main setting of Wolverine & the X-Men, Avengers: Earth’s Mightiest Heroes, Hulk Vs. and Thor: Tales of Asgard.|
|Earth-10022||Planet Hulk (February 2, 2010)||Universe in which the Planet Hulk film takes place.|
|Earth-65||Edge of Spider-Verse #2 (Nov. 2014)||Home universe of Spider-Woman (Gwen Stacy).|
|Earth-67||The Power of Dr. Octopus (Sept 9, 1967)||Setting of the 1967 Spider-Man series.|
|Earth-9907||Avengers Next #10 (July 1999)||An alternate world in which the Red Skull survived his last battle with Captain America and successfully led the Third Reich into total world domination.|
|Earth-8311||Marvel Tails #1 (1983)||The Larval Universe is the home reality of Peter Porker, the Spectacular Spider-Ham and various intelligent, talking, anthropomorphic funny animal parodies of the Marvel characters.|
|Earth-928||Spider-Man 2099 #1 (1992)||An alternate future of Earth-616 set in 2099.|
|Earth-982||What If? (vol. 2) #105 (1998)||The setting of MC2.|
|Earth-2149||Ultimate Fantastic Four #21 (2005)||Reality of the original Marvel Zombies series.|
|Earth-712||Avengers #85–86 (Feb.–March 1971)||Home universe of the Squadron Supreme.|
|Earth-58163||House of M (June—Nov. 2005)||An alternate version of Earth-616 created by Wanda Maximoff.|
|Earth-1287||Strikeforce: Morituri #01 (1986)||Reality where a scientist named Dr. Kimmo Tuolema in 2072 discovers a process that can provide humans with superhuman powers in order to fight back a ravaging alien invasion that started in 2069.|
|Earth-88194||Dr. Zero #1 (1988)||The setting of Shadowline.|
|Earth-811||Uncanny X-Men #141 (1981)||The setting of Days of Future Past.|
|Earth-45828||Razorline: The First Cut #1 (Sept. 1993)||The setting of Clive Barker’s Razorline imprint.|
|Earth-148611||Star Brand #1 (Oct. 1986)||The setting of the New Universe.|
|Earth-807128||Wolverine (vol. 3) #66 (Aug. 2008)||The setting of Old Man Logan.|
|Earth-90214||Spider-Man Noir #1 (Feb. 2009)||The setting of Marvel Noir, a universe in the 1920s–1930s that has few superhumans who developed superpowers.|
|Earth-31117||Captain America #17-20 (2003 – 2004)||An alternate version in which Nazi Germany won World War II, caused by an unknown disturbance in the timestream.|
|Earth-1218||Marvel Team-Up #137 (Jan. 1984)||The setting of our reality, in which superheroes and supervillains only exist as fictional characters in mainstream media and popular culture.|
|Earth-98140||Alien Legion #1 (April 1984)||Main setting of the Alien Legion.|
|Earth-9997||Earth X #0 (Jan. 1999)||The setting of Earth X.|
|Earth-7642||Superman vs. the Amazing Spider-Man #1 (Jan. 1976)||A reality where Marvel and DC characters coexist with one another.|
|Earth-93060||Hardcase #1 (June 1993)||Main setting of the Ultraverse.|
|Earth-8116||Epic Illustrated #1 (March 1980)||The setting of Dreadstar.|
|Earth-13122||Lego Marvel Super Heroes (22 October 2013)||Marvel Legoverse; the main setting of Lego Marvel videogames Lego Marvel Super Heroes, Lego Marvel’s Avengers, and Lego Marvel Super Heroes 2.|
|Earth-1048||Spider-Man (2018 video game) (Sept. 7 2018)||Also known as the Gameverse; the main setting of the Insomniac’s Marvel PlayStation games.|
|Earth-2301||Marvel Mangaverse: New Dawn (March 2002)||Also known as the Mangaverse, an interpretation of Marvel characters through a manga format and aesthetic.|
|Earth-30847||X-Men vs. Street Fighter (1996)||Main setting of the Marvel vs. Capcom video game series.|
|Earth-295||X-Men: Alpha (January 1995)||Also known as the Age of Apocalypse.|
|Earth-1226||M.O.D.O.K. (May 2021)||The Setting of M.O.D.O.K. TV series. It doesn’t take place in the MCU.|
How could the MCU fit into Marvel’s Multiverse?
The integration of the MCU in the Multiverse was a question that arose back when Iron Man first appeared on the screen. And while a lot of people thought it would be impossible, the MCU has – in fact – already been integrated into the Multiverse.
Namely, the MCU’s reality has retroactively been labeled as Earth-199999 and has been integrated into the larger Multiverse as an alternative Earth to the primary Earth-616. This means that the MCU is, officially, part of the larger Multiversal canon.