Also, speaking as a middle school educator, what I can do with class numbers depends so much on what the students come prepared to do.

I currently teach 38 students in my "advanced" math course. These are the students that are taking high school Algebra I for in the 8th grade for high school credit. Managing 38 in that class is just fine. I have more to grade than my typical classes of 30 or less students, and giving individual feedback takes more time and effort on my part.

   The kids taking the high school algebra in the 8th grade come to me as high-achievers that are ready and willing to put in an effort to learn.   Although it's at a faster pace than the typical 8th grade course, and has much higher expectations, it's uncommon for students to carry a grade less than a "B" and management issues are minimal for me that hour.  

I also teach a math support class, we call it the "Math Lab". The program at our school has been recognized by Michigan Association of School Boards as exemplary. This is mainly because we cap the class limit at 15 students and also have two dedicated classroom para-pro's (classroom adult aides) to assist me. In this class, management of the smaller number of 15 can be very difficult. I have to focus much of my attention on the student behaviors and motivating them before helping them with math.

Having more than 15 at times can be ridiculous. Generally, my Math Lab students despise math and do not come to me with very limited math goals and skills. In order to motivate, inspire and educate my Math Lab students attempts at adding beyond fifteen rarely works. As the year progresses, and student "buy-in" improves, I can add more, but I've never been able to justify having close to the 38 students that I have in my "advanced" math class.