Early Decision rules cause a lot of confusion. Here is a clear breakdown of what you can and cannot do.
One ED School at a Time
Early Decision is a binding commitment, if admitted, you must enroll and withdraw all other applications. By definition, you can only make one binding commitment at a time, so you can only apply Early Decision to one school. Applying ED to multiple schools simultaneously would create contradictory binding commitments and is explicitly prohibited by all ED agreements.
ED and EA Simultaneously
While you can only have one ED application active at a time, you can apply Early Action to other schools simultaneously, with one important exception. If your ED school or another target school offers Restrictive Early Action (REA), applying to them restricts your ability to apply EA elsewhere at other private schools. For example: you can apply ED to Dartmouth and EA to UVA (a public university) at the same time. You cannot apply ED to Dartmouth and REA to Harvard simultaneously, because REA restricts simultaneous private school EA/ED applications.
Early Decision II: The January Option
Many schools offer an Early Decision II option with deadlines around January 1–15 and decisions in February. ED II provides a second opportunity for a binding early application, typically used by students who were not admitted in their ED I round and have identified a new first-choice school they are ready to commit to. ED II carries the same binding commitment as ED I.
Strategic Use of Your ED Application
Your ED application is one of your most powerful admissions tools, use it on your genuine first choice. Applying ED strategically (to maximize your statistical advantage rather than because the school is truly your first choice) often ends in regret when you are admitted to a school you're not excited about and have no ability to compare financial aid packages.