Class size depends on the school.
If you are teaching in a private school, you will probably not have more than about 20 students in a class. Sometimes a class can be as small as two or three students.
If you are teaching in a public school, your classes might have as many as 50 students. If this is the case you will probably be provided with an assistant or a co-teacher.
If you are teaching in a university or college, your class sizes could be quite a bit bigger. Again, you will probably be provided with an assistant or a co-teacher.