Multiverse

The Multiverse is an infinitely vast array of mostly independent universes, divided into groupings or "families" of 52 each. Transport between sets of 52 is only possible through the destruction of one of the universes at both ends of the connection.

XSI Multiverse
The primary "family" of universes, also commonly referred to itself as a multiverse, is designated with the prefix XSI, and is the one Bronze Mettle takes place in. This multiverse features a "Spirit World", which is the same across all 52 universes.

Earth 1

The Earth where SI arrived, and the one where most of the story takes place.

Earth 2

The Earth in which Zoom, among others, reside. In the timeline before SI's arrival, this Earth was the one with the most interaction with Earth 1.

Earth 3

The Earth where the Outsider arrived, and subsequently founded the Squadron Supreme. Notably, this Earth is near-identical to Earth 1 in many ways.

Earth 5

An Earth where after the rogue rampage of a supposed hero named "Tighten", the Superhero Relocation Act was enacted, ending the Age of Heroes for almost twenty years.

Earth 6

An Earth where the rise of so-called "Evos" is directly connected with the cycle of solar eclipses.

Earth 9

This Earth's Superman was murdered by its Earth's Lex Luthor in 1934. But his memory would later inspire several nations to pursue genetic engineering, leading to the quiet horror of the Eugenics War in the late 1980s through early 2000s.

Earth 10

The Earth where Doc Future, Flicker, and others reside.

Earth 11

The dangers, the powers... This one goes up to Eleven.

Earth 16

An Earth superficially similar to Earth 1, but with a Young Avengers team, and more prominent division between generations of heroes.

Earth 17

Destroyed by the Anti-Mobius in the process of travelling to the XSI multiverse. This Earth's planet earth was rescued by Earth 1's Hal Jordan, and transported back to his home Earth.

Earth 18

An Earth on which the Age of Heroes began in the year 1602.

Earth 20

The earth is defended by a rebel faction of alien Gem warriors against an empire of their own kind.

Earth 21

Home Earth of Raava and the Avatar. The Earth most in-tune with the Spirit World.

Earth 23

An Earth where hyper-intelligent apes rose to power in the year 1963.

Earth 29

The Age of Heroes didn't start until the year 2099. Crime runs rampant on an earth with more technology than goodwill.

Earth 52

A war between Gorilla City and the Inhumans devastates the earth. True heroes are few and far between, and die too soon.

Earth Prime
The true, single Earth Prime will never be accessed by any other universe on this list. It is the one where you live. Whenever another universe tries to access Earth Prime, a duplicate Earth Prime is created which is interacted with instead of the original. Thus, unless deliberately done, multiple incursions upon Earth Prime will not affect each other, as they will be interacting with different Earth Primes.

See Also:
Multiverse, on Arrowverse Wiki