Planes of the multiverse are effectively infinite. Indeed, many planes have layers
which are also infinite. They are grouped to simplify our understanding of how they relate to one another.
The Great Beyond attempts to discuss the relationships between the planes of the Inner Sphere, the Transitive planes, and those of the Outer Sphere.
More information about types or groups of planes can be found here:
Demiplanes are finite. Most demiplanes are created by powerful magic
and often temporary—they are sometimes made permanent by further magical processes—though they may also be created by natural processes. Demiplane
discusses the nature and properties of demiplanes.
More information about some of the notable demiplanes can be found here: