There's no single answer to the average number of students per class at UC Berkeley. The average class size varies dramatically depending on the subject, the level of the course (introductory vs. advanced), the type of course (lecture, seminar, lab), and even the specific instructor.
Some introductory lecture courses might have hundreds of students, while advanced seminars might have only a dozen. Therefore, any single number would be misleading. UC Berkeley doesn't publish a single overall average class size.