Each quarter, you can enroll in a minimum of 12 units and a maximum of 20 units (though we would recommend 12-15 units for your first quarter). However, you can attend the first few meetings of most courses without being officially enrolled. This gives you the opportunity to "shop" many possible classes during the first week, and then pick the ones that work best for you.
Attending a larger number of courses that first week gives you options when putting together your final study list--you can evaluate the syllabi, the pace of the course, the density of the reading, and then decide which courses work best together. Visiting classes can also be a way to sample a field, make contact with a professor who interests you, or look ahead to what you might take in the future.
While you can "shop" most classes without officially enrolling in them, there are some notable exceptions: Introductory Seminars, PWR courses, and some limited-enrollment courses, for example. We recommend that you contact the instructor before trying to sit in on one of these classes.
Officially, you can add/drop classes through the end of the third week of classes. Practically, however, while you can drop all the way through the third week, you should probably add your classes by the end of the first week of classes or the beginning of the second at the latest. Professors are often very reluctant to allow students to add a class that they have not been attending. The quarter system moves very quickly, and Stanford courses cover a lot of material. Even if you’re just checking out a course, you should be doing the work. Otherwise you may find yourself uncomfortably behind by the time you make the decision to stay in the course.