2-per-person (round down) sounds about right. We have 2.5 and the 2nd full-bath (in the basement) is rarely used. I'm flabbergasted to know that, as an upper-middle-class Canadian in a 5-person family, my home would be below average compared to a new American development. One-person-per-bathroom is madness.

Stephen Colbert said it best: "One for dinner-guests and one you poop in".