<p>To add to the comment by ucbalumnus: My university will not cancel a class for Ph.D. students provided that there are at least 5 students enrolled. Classes with 10 students or so are common at that level. At the undergrad level, the enrollment needed to guarantee that a class will be offered is 15–but in practice, classes that are essential to a degree program may be offered with as few as 3 students.</p>
<p>Also, most universities that have 500-student general chemistry classes wish they had enough students in advanced chemistry courses to make up 20 sections of 25 students! They are more likely to have enough students in advanced chemistry to make a single 25-person section.</p>