Magic number - schmagic number! If you have a group of highly motivated kids you can teach to as large a class as you can fit in a room. I'm not really disagreeing with anyone, sure there are circumstances where its better to have smaller classes - and God knows its easier on the teacher. But I like what Joan Reinthaler said. If you've got a group of kids that have a good background and are willing to work, then the more the better. And if you've got a bunch of kids with serious weaknesses, then there's no way you're going to be able to teach effectively to more than just a few. To me its as simple as that. But then again I'm a simple person - I still teach like they did in the dark ages - lecture, examples, practice. Funny thing is my students manage to pass - most even do well.
Oh, and in case anyone is interested, last night I taught a review session to well over two hundred students, every one of which left with a better understanding of intermediate algebra/precalculus. Again, it's a matter of motivation. These were college freshmen and sophomores who were attending voluntarily, and scared to death of failing Test 2 - so they were definitely a motivated bunch.