The proportions of this recipe are not rigid, so you can add a little more or a little less of something, according to your taste.
For the dried beans, use whatever fairly large white beans you can get. A good French variety is 'lingot du nord'.
For the duck confit, if you are using legs, allow 2 per person.
For the sausage, use a good variety that is available where you live (I prefer using smoked sausage, but this is a matter of taste).
Cassoulet is one of those dishes that just gets better each time you reheat it, so don't hesitate to make a large quantity for reheating on another day or for freezing.