So I went and put one in every bedroom like the old laundry in previous sims but when doing laundry they only empty one per load so I guess I only need one?
Not necessary for this iteration of laundry. You'll only need one, less you have a big sim family in which SimGuruGraham suggested one for each sim. Though, I played three sims in a household with one hamper and only have to do laundry maybe every 4 to 5 sim days at best?