You could add the whole carton of black beans if you like. I didn't want to add too much that's why I decided not to put the whole carton, however the whole carton should be fine.
This stew will result in 2 good adult portions but it could serve 3 people as well, just make sure it's not the only thing which is being served as on it's own may not be that filling.
If it feels too thick just add some water, however it should be the consistency you see in the pictures (pretty thick).