The particle grid emitter is used to generate particles and store them into a particle group of choice. It can generate any Effex particle type.
The emitter itself does not know anything about the target particle group.

However, the particle emitter is completely based on a grid (hence its name), so the global voxel size defines how many particles are generated.
Currently it generates 8 particles per cell. This is the particle density that is most useful for the algorithm used in the liquid simulations (FLIP) and at this time cannot be altered (also to guarantee a certain simulation quality).