This snippet from a Blue names the realms in each cluster
There are 5 clusters in the North America region and also one OCE cluster [Arugal/Felstriker/Yojamba] :
PvP West (Anathema/Whitemane/etc)
PvE East (Ashkandi/Mankrik/etc)
PvE West (Atiesh/Old Blanchy/etc)
PvP East 1 (Benediction/Incendius/etc)
PvP East 2 (Earthfury/Skeram/etc)
The PvP West and PvE East Clusters receive free transfers and are thus the largest in the region. I play on the smaller PvP East 1 realm as Horde on Incendius.