Side note, I will be referring to discussion on this topic about the number of students that attend Hogwarts since it is what got my mind thinking about this.
I have been giving a lot of thought to the set-up of Hogwarts, and while reading the wonderful discussion on the previously mentioned topic, it got me thinking, "How conceivable is it that there are more than just fourteen dorm rooms, seven for boy and seven for girls, for each House?"
It's been well-established, in my opinion, that there can't be exactly five boys and five girls for each House every year, but perhaps that is simply how many fit in a dorm. I suppose it could make sense that the rooms expand and shrink depending on how many students need to sleep there, but even in the Magical world, you can only expect so many teenagers to share a single sleeping area.
As discussed more in the previous topic, Harry's class may have been exceptionally small for a large school which normally has thousands of students, but this idea can work for or against that idea. If Harry's class was tiny because of the war but other years are larger, especially if this is the Magical equivalent of the Baby Boomers, then there could be over ten people in one room. For the sake of everyone's sanity, it would make sense to split them in half.
Then again, if Harry's class size was actually of the norm, this idea could make up for some obviously missing students. Maybe the reason Hermione seemed to only have two roommates was because there was another dorm of Gryffindor girls in her year.
This would also help to manage a larger body of students to come long after the Trio has left Hogwarts. If the first defeat of Voldemort didn't cause a Baby Boom, then surely the second and final one will. This is especially important for Next Gen fics where the (plausibly second) Baby Boom kids will be attending Hogwarts. Especially with prejudice against Slytherin House, the other three Houses could experience an increase of members. If Hogwarts accepts about sixty students every year (basing off Harry's year having a small student body of around forty or so), and say about six go to Slytherin, the other Houses could expect about eighteen students each. In my mind, at least, instead of jamming nine students into one dorm room, why not split them into five and four? Even if the number is split evenly among all four Houses, that's still seven or eight people in one dorm. Considering the largest number of people sharing a dorm in the books is five, that's a lot of teenagers in one room.
(Or maybe this idea is glaringly American and the British are a lot more comfortable with sharing sleeping quarters...)
As far as I know of my Canon knowledge (which isn't as fantastically thorough as I know other's are), JKR never states that there is only one dorm for each year's genders. This is just an idea that could help solve the discrepancy between Hogwarts having 'thousands of students' but only eight Gryffindor students in 1991. I would love thoughts on this proposal, so please let me know how believable multiple dorms are at Hogwarts.