The number of holes in salt and pepper shakers is not set in stone. It varies in different places. In the United States, there are fewer holes in salt shakers than in pepper shakers because salt is generally considered bad for the health.
The availability of either salt or pepper may also affect the number of holes put in their respective shakers. In some parts of Europe, pepper shakers have fewer holes because spices are exotic and more expensive there. The reverse is true in other places, where salt is a rare and highly prized commodity. The size of the grains can also be a factor. The number of holes will determine how well the grains of salt pour.