There is no set number of layers within the Abyss. They are always being created and destroyed by the entities spewing forth from the heart of this centre of destruction. Those powerful enough to take control over an area and maintain it are known as demon princes. Each area is bent to the will of the Prince controlling it and the landscape scarred by their influence.
The various layers of the Abyss are connected via various portals as well as hidden tunnels and paths. Some of them border the river Styx, allowing them access not only to other layers byt also some of the other lower planes. These are the ones that are most active in the Blood War and who send hordes of demons to the Nine Hells. Almost all layers are wastelands of some sorts. They might be the hunting forests of Yeenoghu, but even so, they are overgrown and tangled chaos rather than peaceful.