These steel thin nylock fine thread nuts are sold in packs of 50, with the exception of the 5/8"-18's which come in packs of 25.
Also known as nyloc nuts, they have a ring of nylon inside the thread. Nylocs are designed to increase locking force and prevent the nut from coming loose.
Several sizes of nylocks are available for your application.
Catalog Correction-94004210 is a 1/4-28 thread, NOT 1/4-20.