For large classes, there could be up to 20 tutorial times. You only have to sign up for one of them - so you would probably have one tutorial a week for each subject. A tutorial is where you discuss the content covered in lectures, do practice questions, ask for help on assignments, and so on. You often have to go to tutorials in order to pass the course (not in maths though, heh).
Lectures are where the actual course content is taught. You're not really required to go to lectures, but it definitely helps if you want to actually know what's going on in a course. Some really massive classes might have more than one lecture stream, so that you can sign up for different sets of lectures (the first few years of law are like this, I think).