system
Given the following post, respond as if you're commenting on Reddit:
human
[DC/Marvel] How do the multiverses work?

For example, are there only like four Marvel universes within its multiverse, such as the Avengers, X-Men and Spiderman and Deadpool etc? Is it also the same in DC? How does it generally work?
gpt
The Earth of the main Marvel universe is called Earth-616. The Avengers, Fantastic Four, Spider-Man, X-Men and so on all exist within that universe. The vast majority of comic books published by Marvel comics that don't carry a specific imprint (like the Ultimate line which takes place on Earth-1610) take place on this Earth, or at least in the universe in which that Earth exists.

Vast numbers of alternate Earths/Universes exist. For instance, the early 90's Iron Man, Fantastic Four and Hulk cartoons take place on Earth-534834. Earth-10005 is where the X-Men movies produced by 20th Century Fox exist. 

The current Marvel Cinematic Universe takes place on Earth-199999. It was intended during production to include the Oscorp Tower from the Amazing Spider-Man in the New York skyline in order to indicate that this new series also takes place on Earth-199999. Instead, this takes place on Earth-120703. Similarly, most Marvel-based movies and TV shows take place on their own Earths unless there's a clear crossover. 

Each Earth is often very similar or wildly different to Earth-616. Sometimes a small difference can lead to a big change, like the Starks having a baby girl instead of a boy. On this Earth, Steve Rogers and Natasha Stark were a couple and this prevented the Civil War from taking place completely.

On Earth-6078, Ben Parker lived and his wife May died, leading to a very different Spider-Man more interested in fame than responsibility.

And on Earth-1218, superheroes only exist in comic books, TV and movies. In this universe, the idea of other universes is something comic book writers made up. Of course, this universe is extremely boring and no sane person would want to live there.

Although it does not have a number, and how all of their constant crises affect this is unknown, but in theory the DC universe is a universe that exists within the Marvel multiverse.