How many classes should you take? It depends. It depends on the classes, it depends on the workload, it depends on your other commitments, it depends on who you are and how you learn. This decision is best finalized after classes start--the ideal approach is to attend more classes than you expect to take, and by the end of Week 1, decide which of those classes make the best combination for you that quarter.
You will need to sign up for at least 12 units by the end of the first day of classes.