Quantity is up to you
The better the bread, the better the crumbs; I like whole grain. Cut the bread into ½-inch-thick slices, leaving the crust on. Cut the slices into cubes and then spread them in an even layer on a baking sheet (or more than 1 pan, if making a lot; a 12-ounce loaf should fit onto 1 pan).
Heat the oven to its lowest setting, usually about 250°. Bake the cubes until they are fully dry, but not browned. This could take an hour or more, depending on the bread’s moisture and density.
Cool fully and then process into crumbs by pulsing in a food processor. The goal is small crumbs more or less the same size, though some bigger ones are fine—think Grape-Nuts. You want to avoid too much fine powder, however, so stop once or twice and pour off the finer crumbs or shake through a colander and then continue to crush the remaining big pieces.
Store the crumbs in an airtight container. If fully dry, they’ll stay fresh for a few weeks.