(A/The) purpose of ZRemesher is to automatically retopologize your model to a much lower polycount using predominantly quad polygons. This allows it to be subdivided to higher levels to preserve detail but lowered to a smaller subdivision level for larger scale operations like movement, smoothing and major form changes.
The number 5 in the polycount refers to 5k (5000) polygons. Max on the slider is 100k. You can target up to 250k through the Gizmo:Remesh by ZRemesher option.
As for say going from 9.6M to 5.8M, not with a ZRemesher control directly. More likely to ZRemesh to a lower level and then subdivide up to the target 5.8M, knowing that each subdivision multiplies polycount by near enough to 4. So ZRemeshing 9.6M to about 5.6k will give 6 subdivision to reach about 5.8M.
You could Decimate as suggested to give somewhat more control over polycounts but the mesh will be comprised only of triangles which has it’s own uses and limitations. I replicated your trial, without the move deformation, and got 8.914 at 20%. That is 8.914 million points which is pretty close to 20% of my starting 44.57M points. Your result will vary because of the deformation. Remeshing with Decimation Master or ZRemesher will adjust mesh size and density to preserve detail. Also note that Decimation Master shows estimated counts of both polys and points when adjusting the % decimation slider. Looks like the percentage refers to points. Point count and polycount are almost identical when polys are quads but polycount is about twice point count when polys are triangles. You can adjust the %, poly or point counts. So if you want a specific polycount then adjust that value rather than the % decimation slider.
Check out Intro to Zbrush by Michael Pavlovich. ZRemesher is #45.
