In general, plan to serve 1/4 to 1 pound of shrimp for each person. The amount may be scaled up or down depending on the accompaniments being served. For example, when making a shrimp boil with potatoes, corn and sausage, 1/2 pound of shell-on shrimp per person may be sufficient.
Keep in mind that shell-on shrimp loses some of its weight once the shells are removed. If keeping costs down is a concern, offset the relatively high cost of shrimp by building a menu that includes boiled shrimp as well as other less expensive items, such as salad, bread, pasta or rice.