When you see 16-20 or 26-30 on a bag of shrimp it tells you about how many shrimp you will get in a pound. It's a great way to judge how many shrimp per person. If a recipe calls for 8 ounces of shrimp (1/2 pound) and you have a bag of 16-20 size shrimp, you know you'll need 8 - 10 shrimp for that recipe.