Your decisions are made privately and with no outside pressure. It is to your advantage to fill out your list with the most desirable programs on top. Don't fill out your list based on the order in which you think you'll be selected. You have no way of knowing which programs put you high on their list. You can rank as many programs as you like.
Most students rank several programs in their specialties of choice.
Your decisions are made privately and with no outside pressure. Every year, there is a small number of students who do not match. If this happens to you, don't panic. It doesn't mean that you'll never be accepted into a residency program. You have to take immediate action.