There is no upper end on sharpness. Sharper is always better. A knapped edge on glass or obsidian is many times sharper than any conventional metal blade and they aren't perfect. But there are practical limits for how much time and effort I'm willing to dedicate to getting a sharper edge.

If a knife edge won't skate on my thumbnail, it digs in, it is sharp enough for most uses.

Materials used for knife blades are a compromise between, toughness, flexibility and shock resistance and the ability to be sharpened to an edge and hold that edge. Most common alloys used for knives won't hold a very fine edge in practical use. Those alloys, or materials, like the ceramics, that are hard enough to hold a very fine edge are often so brittle that the blades can fail catastrophically in use. Some are so brittle they can snap if dropped. Extreme cold can induce similar brittleness in some otherwise practical alloys.