T.F.
The girls are 12 yrs old. If your daughter has problems in a group setting and does better one on one, do not do a sleepover. Keep in mind who the drama makers are and consider who you invite. Make your behavior expectations known (to girls and parents) ahead of time so no one is surprised if you send someone home.
Do not have an odd number. Like the pp said, keep even numbers so there is no odd man out.
At 12, they should be able to get along without supervision 24/7 (you in the room with them). Of course, you should be around and available but allow them some space to be social with each other. They are still learning social skills. Part of that learning is working through their own drama and personalities.
We did 2 parties with a limo and when you use a limo, you automatically have a limited # of attendees! One party ended up at our house for a sleepover where we had pizza, made a craft , mani/pedis, movie and breakfast. I think my daughter was around 10 at that time.
For daughter's 12'th bday, we did limo party again, each time the limo picked each girl up at her home. On the 12th bday, they were dropped off at an upscale hotel located in an upscale shopping center where I had already prepared a suite for them. The girls 6 total, slept in 1 room and I had the other room of the suite. I slept with my door closed and allowed them some space. This time, they went swimming, etc. Then we got all dressed up for dinner, walked to a nice restaurant for dinner, walked to an ice cream shoppe for a treat, back to hotel for movies, late night swim, room service snacks, breakfast brought in the next morning and everyone gone by 10am.
It was a time of year that the hotel was kindof slow so we got a suite in an area of the hotel where there were no immediate guests so no one was bothered. I STILL used this experience for them to be aware of surroundings and how other people may be sleeping, etc so they would be considerate of other guests and I expected them to act like young ladies vs young girls running wild.
Again, you mention your daughter being emotional and girl drama... keep in mind that your daughter may not be ready for a party like this and that is ok. This is a prime age for girl drama. If I knew from the get go that my daughter might not be emotionally mature enough for a party like this, I would not have the party.
As for the invitees... you owe no explanations as to who is invited. At this age, a lot of people either have a huge party or a small party. Keep a balance of the girls based on age and emotional maturity.
Good luck and time flies!