There is not an equilibrium number of dislocations within a crystal, although they do occur in all crystals.
Glide plane is the plane to extra half plane and is the plane of slip or glide.
Glide planes and slip directions are crystallographically controlled. That is, as stress builds up the planes and directions of weakest bonds slip. This is important because it means dislocation creep can result in a preferred orientation of the crystallographic axes which, in turn, provides a way of identifying dislocation creep in thin-section. In 3-D dislocations can form loops with edge dislocations in front and back and screw dislocations on the sides. This is important because it results in undulatory extinction in deformed xtals.