You didn't mention how your party is broken up, so depending on that, 12 is doable.
In Jan 2001, we had 11, with one family of 4 in one of the rooms upstairs, a family of 3 in the other room, and my family in the master bedroom. However, my children were young, and one was in a pack and play and the other slept with us. That makes a total of 11 in our group.
One of the bedrooms upstairs is HUGE and you could easily put a cot in there or sleep on the floor. Not to mention that the couch in the living room pulls out to sleep 2.
As long as everyone doesn't mind bunking together, here is how I would do it:
You, DW, and 2 1/2 dd in Master
BIL, SIL and their 10yr DD in upstairs smaller bedroom
Your 19yo DD and her guest, and your 18yo niece and her guest in larger bedroom upstairs.
21yo niece and her dh will have to sleep in the living room. (There is also an extra bath on the first floor).
If the breakouts don't work for you (depends on how happy the teens are about sharing beds) you might consider a different arrangement than the GV. Last May we went with myself, DH, Daughter 25 and her hubby, son 29, his wife and 3 year old, and DS, 19 and his buddy. That's 8 adults and a child. The GV was not available when we made plans (plus I had concerns about privacy) so we reserved a 2 bedroom plus 2 studios. That gave us essentially FOUR bedrooms plus the livingroom of the 2 bedroom (versus the 3 bedrooms of the GV). We still had the big kitchen area of the 2 BDRM for getting together for pizza, etc. Not as much common area as the GV, but still quite a bit. Plus the 2 BDRM added to 2 studios was less points than a GV. That arrangement might be something to consider if you just can't work it all out.
Mike's Mom has something to consider. How much togetherness can your group stand? Sometimes having the chance to get away from each other for a bit (i.e., own villa) makes the rest of the time together less stressful. On the other hand, the GV is spectacular....but you might want to keep the chance to break-up the group in other ways during the day as something to "plan" for.
I would probably opt for the two bedroom and two studios also. I don't consider it a vacation when you have to share a bedroom, even if its with your own kids. This option would offer more privacy, better sleeping arrangements and still give you a pretty fabulous living area.
We did 11 in a GV a few years ago. This included 4 children though, and one of those slept on the floor in his parents room while his sisters shared the second bed in that room. We had only one on the hide-a-bed also. The only thing that was crowded was the sleeping. We really found there to be plenty of room for everything else. That is except for the laundry! The GV laundry room at OKW is smaller than the 1-2 bedroom laundry, and it was noticeable. With 4 kids, there was laundry every day.
In 1999 we opted for a 2 BR plus 2 studios rather than a GV. It was about the same number of points for more beds, more privacy and more free tickets as this was our last hurrah for the free tickets. In a way I regret it as it might be my most reasonable option to try a GV but the frugal part of me couldn't see paying the same for less.
We Did 12 in a GV. Three families, each had a bedroom with parents and children and one single on pull out couch. Now the person in living room had least privacy amd had to get up each am as the GV awaken but he had the closest and bath in the front hall for all his belongings. It really wasn't bad and the GV is so great that I would do it again and not get separate villas. You feel so special in a GV!!
We'll have 12 in a GV next August. My husband and I have the master bedroom, by b-i-l and s-i-l have one of the other bedrooms, and the 8 teens/young adults can sleep wherever they want.
The plans right now are to have the girls (21, 17, 12) share one bedroom, and the boys (20, 20, 17, 16, 15) to all camp out in the living room with air mattresses. They love the idea, and they'll be up until all hours playing cards anyway - it's one of our family vacation traditions.
I've had 10 in a GV in the past. It never felt crowded, especially since the adults had their own bedroom to retreat to when we were experiencing a little too much togetherness. The kids always wanted to be around each other, and we didn't spend enough time in the villa to have that ever become a problem.
It's vacation. We all adapt and have a good time. All I have to do to smile is remember the five of us in one room at Dixie Landings.