Growing up in Rome, our rule was 100g per person, and 100g for the pot if it's just a light sauce.

Basically, a box fed about 5 people.

That seems like a pretty good rule of thumb. I tend to eat a bit less than that. If I'm planning for a group doing some activity like strenuous hiking, I'll plan for a bit more. But 4-5 people to a box/pound sounds about right.