6-8k stones are the most common as they are the best value. You get a decent edge, and the stones are usually around 40$ for a decent one. Right now its unknown the highest grit possible, primarily because with higher and higher grit, manufacturing becomes exponentially more expensive.

Grit is sorted through filters, the finer the filters the faster they clog, and the finer the grit, the more likely they are to float away, or do not have the inertia required to pass it.

There are 2 ways a blade dulls.

Abrasion: abrasive particles in the material rounds the edge of the blade by scraping off microscopic bits of steel.

Microscopic chipping: In this case the steel fracture and pieces break off the edge, revealing a duller edge.

Scratches act as weak points for the latter. Scratches form a sawtooth like shape. People say these "micro-serations" aid in cutting and are even desirable, but I would disagree. The peaks of the teeth break off more easily and the deep grooves provide further weak points. They will dull rather quickly.

Very hard steels (also depends on the type, and the knife, and best treatment, and edge geometry ) will dull significantly from microchipping. If the edge is as robust as possible, it will remain much sharper. The finer the surface, the more robust the edge is.

It gets wayyy more complicated considering edge geometry (just a 1 degree difference can stabilize a chippy edge), natural stones and how they are completely different, glossy vs cloudy finishes (cloudy is better), slurry, omnidirectional scratches patterns, etc.