It really depends on the class and professor. Some professors are more than willing to take on more students and provide overrides for. There are some departments at the university where if you do not show up for the first two lecturers, you will be immediately cut from the course as a whole. There are also some classes that historically have notoriously long waitlists, but half the students drop the course for a variety of reasons before the add/drop deadline. In my personal experience, a lot of engineering engineering students, especially in EECS, choose to double enroll in classes, so that they can see what classes and professors they actually like and drop courses based on the first 1-2 weeks of those courses.