I have 20-21 students in the room, depending on the subject.
I usually change the room arrangement every 2 weeks. That way no one has to sit in the front/back too long, or be stuck by "that kid."
Anyway, when I do put them in groups, I sometimes do groups of 4 or 5. I usually expect the voice level to naturally be raised though and sometimes I don't leave it that way for long. I also usually tell the groups that if there is too much chatting I will move a desk or two away, and they definitely don't want to be the "odd desk out."
Another way I arrange them to be "together" but not necessarily in such close quarters is to arrange them in "L-shaped groups". That way they are still a group, but I can walk up to each one of their desks to see them individually if needed. Make sense?