I almost always figure a 4 ounce serving of beans. I insure a 4 ounce serving by using Chinet 3 Compartment Plates---that have a 4 ounce serving area for side dishes, and I always put a 4 ounce serving ladle in the beans. This way even in an open buffet line I can control serving size.
I never use canned beans----but we won't start that arguement again----if I was using canned beans I would read the volumn of the can --convert it to ounces ---then divide the total ounces by 4 to determine number of 4 ounce servings.
Bean Arithmetic for dry beans : One pound of beans equals 2 cups of beans. 3 pounds or 6 cups of beans will make 50 four ounce servings. So for 600 people I would use a mimimum of 36 pounds of dry beans. I would add 12 pounds of bacon or other meat and several ham shanks.