i'm thinking back to when our household was 4 people-2 f/t working, 2 f/t in school. 8 loads a week was the minimum i was running-
2 loads of towels minimum-everyone showered/bathed AT LEAST once a day/twice if yard work/other physical activity/going out in the evening, so that was at least 28 bath towels and 28 washrags at the lowest end.
1 load of bedding
2 loads of 'sports wear'- kid's gym clothes, sweat pants we wore as comfies...
2 loads of whites minimum
2 loads of colors minimum
1 load of 'dedicates' minimum (not nesc. delicate clothing but stuff i prefer to do as a separate load in colder water and initially air dry b/c it's prone to shrinking or pilling up).
winter required more loads b/c people layerd up clothing wise, were more apt to wear socks, needed gloves, beanies, scarves run every week or so and migrated to wearing their fleece pajamas which required separate laundering from traditional sleepwear (fleece is a hair/dust magnate so i wash it separately to keep it from transferring fuzz and such to other clothing).
THAT SAID-
i look at that bathroom and it hurts. i went through the same battles with my kids over how dh and i have a level of expectation on how ALL rooms in our homes be maintained, their rooms included-despite their desires, those rooms are in the home dh and i own and they WILL be maintained in a safe, healthy and accessible manner (a former friend, a fire fighter was injured responding to a fire in a home that had a bedroom floor clutter with clothing and odds and ends, i once wrenched my back tripping over items in one of my kid's rooms-that established the 'clear floor' policy).
op-only you know what collateral you hold with your teens. are they the type that will respect your directives (or your dh's)? if not then there have to be some type of consequences put in place. for some it's the loss of the clothing, for others it's the loss of some type of privilege. you know best what works with your kiddos-but you have to put something in place b/c either you have a whole lot more bathrooms such that you can afford one entire tub being a hamper or you are stuck clearing that tub repeatedly (and praying noone outside the household ventures in there).
good luck-let us know what happens.