My kids each applied to about a dozen. This approach certainly doesn’t make sense for everybody, but it can make a lot of sense for somebody who really wants to attend a highly selective school like Yale. Even for highly qualified students, the single-digit acceptance rates make it very difficult to predict how admissions will go at those schools. If you look at results threads here on CC, you will often see kids who got into one super-selective school but not another, with no obvious reasons. Another similar kid gets the opposite result. It’s not unusual to see a kid apply to six or seven super-selective schools, and get admitted by just one of them. If he had shortened his list by one or two, he might be at a match or safety–not a tragedy at all, but just not the result he would have preferred.
People who need to compare financial aid offers may also need to apply to a larger number of schools.