Hi! The plugin is simply a type of Scene in GB Studio and is based on those limits, which you can read about in the GB Studio docs.
In the example scene that comes with the plugin, the background uses the maximum number of tiles (192), but that is only one area type. If you take a look at my (very unfinished) jam game, Abducted, there’s forests, caves, and a more plain warehouse style areas, each using different tilesets.
I don’t know how many full backgrounds you could fit into a GB Studio game, but there are projects with literally hundreds of them out there so just play around.