In both the comics (Years 1 through 5) and Injustice 2 and both games, there are references to at least three Earths. I can remember a few off the top of my head:
- Injustice 1 Game: Earth where the "Heros" come from. My assumption is that this is Earth 1
- Injustice 1 Game: Earth where the story takes place and the Regime is in charge at the beginning
- Injustice Year 2: Dr. Fate saves Black Canary by taking her to another Earth where she meets an alternate Oliver Queen. It's assumed this is the same Oliver in Injustice 2.
- Injustice 2: Reverse Flash mentions his future is messed up because of the Regime. Is he from the same Earth or did he cross dimension barriers as well (like the heroes in the first game)?