A typical portion of cassoulet is about 1/2 cup or around 120 grams. Depending on how hungry your diners are, you can estimate that each person will need between 120 and 240 grams of cassoulet.  As it is a very rich and filling dish, it's usually best to err on the low side so that your guests don't feel overly full afterward.  Additionally, if your recipe includes large portions of meats or sausages, it may be a good idea to serve slightly less cassoulet per person in order to balance out the dietary fat content.