There are currently 2 regions in PokeOne, Kanto and Johto. Unova is currently being worked on and is set to be out around the beginning of 2019.
Each region has their own separate trainer level. This is to keep you from steam rolling through battles after completing the first region, because you get to keep all your items and Pokemon when traveling between different regions.
A lot of people ask how do you get to Johto from Kanto and vice versa. You may remember entering a room (Pokemon League Reception) when you were on your way to defeating the Elite Four. You want to go back to that room. The North route takes you to Victory Road, the East route takes you to Kanto, the South route takes you to Johto, and the West route takes you to Mt. Silver (where you can battle Ash.)
A map listing all the routes, cities, caves, and more will be added soon!