Might be potentially infinite universes.
When they're travelling out of the Demon Realm, they are warped to a place where there seemingly is a wall of endless exits. To which the fish guide asked which universe they want to go to (they say Universe 7). They proceed to go I that exit and end up in universe 7. Lot of dimensional travel, lot of references to the Outside world.
This is only potentially cause they didn't confirm or give any information. Additionally, when they say to go to Universe 7, they ask where in Universe 7, they say a number 3. So the exits could be potentially be different points in universe 7.
A guide could easily confirm either one.
Also some are speculating there is one demon realm for all the universes, as it's potentially framed that way.
Again, nothing confirmed. Gonna have to wait for a guidebook or just further episodes