Applying to fewer institutions isn't always ideal, but there are situations when it's necessary. The most apparent rationale for applying to fewer institutions is if you apply for an early decision. In this instance, you can only apply to one institution, and if accepted, you must go. These applications are typically submitted in November, with decisions issued by December, well ahead of ordinary decision application deadlines. If you apply and get accepted to your top early decision school, you will have only applied to one school.