Cooking time and number of servings will vary based on the size of the roast. Figure on roughly a half hour roasting per pound and figure you'll be able to serve 2-3 people per rib. The smallest roast you should do is 2 ribs and the largest possible, a full roast, which will contain both the small/loin end and the large/chuck end will have 7 ribs. You might want to consider 2 loin roasts rather than one full roast if you have a large crowd to decrease the cooking time and also to use just the superior end of the roast.