The USMLE doesn't set a limit on the number of times you can take the Step 1, but your local and state medical boards may as well as your medical school. In New York State the limit is three or five attempts and at SUNY Downstate, the limit is three.
The exam is now administered by appointment through Prometric, which absorbed Sylvan Learning Centers, so it's given year-round. However a student cannot take the Step 1 until 60 days after the last attempt.