In my years of teaching experience I've had the opportunity to conduct class for a very few students at a time.  The theory that "less is more" in the classroom is correct, to a certain extent.  I've taught classes of thirty students and had major difficulty with discipline and student engagement.  But more surprising is the challenge that an educator faces when trying to teach one, two or three students. 

Teaching and tutoring are not the same thing.  A tutor is expected to work one-on-one with the student in a individualized environment for a specified amount of time.  A teacher, however, plans lessons based on learning goals, not evident weakness.  So when a teacher designs a lesson for one student it is a challenge.  Two students aren't much better.  It puts a lot of pressure on the student be fully engaged all the time when they are the sole audience. 

In my experience, teaching three or fewer students at a time just presents a whole new realm of challenges.  For example, what happens if two students are out?  This is not unusual for a class of 25 and of course, the lesson would go on.  But in a class of three, when two students are out it hardly seems worthwhile to continue without them.  Afterall, you will just end up re-teaching the material when they return.  So while it may sound great in theory to only have a few students, in reality, I think that somewhere between 10 and fifteen is ideal.  There is strength in numbers, in this case.