If you use fresh clams, about 48 small hard-shells will be perfect for this recipe. Place the clams in a large pot and cover with 1-1/2 quarts of water. Set pot over medium-heat and bring to a boil, then cover, and reduce to a simmer. Cook until all the clams have opened: about 12 minutes; discard any clams that don't open. Set a strainer over a bowl and line it with cheesecloth or a damp coffee filter; pour the cooking liquid through the strainer. Remove the clams from their shells. Use this liquid where the recipe calls for clam broth.
You can also use a mixture of fresh and canned clams. Just use enough canned clams to make up for what you don't have in fresh.