I have taught classes of 20-25 lots of times, these were usually workshops or interactive discussion classes. I prefer the small group approach or one-to-one training in the lab, its more freeform and requires less planning, and suits my personal style a bit better.
I would stick to anywhere between 10-20, in even numbers if possible. My style of tutoring involves lots of activites and pairing students up or splitting them into groups. So a manageable even number would be ideal.