Realm 1 is the Realm of the Oni and the Dragon. There's the realm which the ninjas reside in.
What are in the other 14 realms?
The fandom wiki website gives a nice overview over the known realms at this time:
List of Realms:
◾Realm of Madness
◾Cursed Realm (Destroyed due to immense flooding from the Endless Sea)
◾Djinjago (Destroyed in consequence to the destruction of the Cursed Realm)
◾Realm of Oni and Dragons
◾6 other unknown realms.
There can you find more information about the single realms too.