I read that some places like Inlingua or ECC pay around 400 bath per hour. However, these classes are kind of small (5-6 students). What about a part time job where you get to teach 30-40 students? Is the pay rate increased for having so many students or is it the same? Would 500 baht per hour be a fair rate for teaching 30 university students? 

Fairness isn't an issue. This is business and it's what you do for a living. If you are doing it for fun, then don't charge at all. Merry Christmas.