Nobody really knows how many planes there truly are. Many believe that there is a finite number of planes, merely more than can ever be counted in the combined lifetime of every being in existence. However, another source notes that the Multiverse is "boundless".
The "Nexus of the Multiverse" is a plane in the center of the Multiverse with a great influence on all other planes.
It is currently unknown where the nexus is located. Being a Nexus is probably connected with a large amount of mana lines flowing into it from other planes. That lets it quickly regenerate its supplies of mana and most likely is a way of how it affects other planes. Its collapse can cause the destruction of the time stream—an abrupt end of the Multiverse's existence and the near-infinite amount of its planes.