I believe the smallest chapter is Tri-Delta, a full pledge class for them is about 25 women, and that meets their full quota.
No, no, no, I am a Tridelta at IU and this is incorrect. Our pledge classes all vary in size; who knows how they've ended up getting so skewed over the years but we currently have 44 seniors, 27 juniors, 30 sophomores, and 45 freshmen.