As an incoming freshman, you can enroll in up to 20 units for Autumn Quarter (though we would recommend a load between 12-15 that first quarter). You will need to sign up for at least 12 units by the end of the first day of classes. However, you can attend the first few meetings of most courses without being officially enrolled (Introductory Seminars and PWR courses are some exceptions). So the first week or so you can attend as many classes as your schedule allows.
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.
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.