This is not my experience at Stanford, nor is it the typical experience I’ve read from others. Instead Stanford, and several other similar schools, try to expand class sizes/numbers/faculty/resources to support increased demand, so students can register with any nearly class they’d like and meet prerequisites without issues of limited spacing. For example, in recent years there has a CS boom at Stanford, becoming Stanford’s most popular major and increasing in size by a factor of >4x during a small number of years. Rather than restrict the major or limit the number of students who can register for intro CS classes, they let intro CS classes grow to record sizes. The article http://www.stanforddaily.com/2013/06/04/cs-popularity-reaches-record-high/ mentions the intro CS class CS106a had 1,817 registered students during the 2012-13 year, which posed a variety of organizational problems including safely fitting so many students in a lecture hall. To avoid similar problems in future years, the article mentions plans to hire more CS faculty and split the class in 2 halves, with a morning and afternoon lecture that cover the same material. Instead of trying to limit CS enrollment, they mention plans to offer a new CS+humanities dual major to better accommodate current student interests, which may further increase enrollment. The only class I can recall ever having limited enrollment involved studying anatomy with human cadavers. They had to limit the number of students to match the number of available cadavers.