As far as the number of away rotations that people do, I've generally heard from fellow interviewees 2-3 as well. I did 3 rotations, and I generally felt that it was the right number.
There's no hard and fast rule on who should write your letters. My suggestion would be to choose away rotations where you can work 1:1 with the most well-known plastic surgery faculty (family names in plastic surgery) so they can be make personal comments about you. Chief/PD letters are commonly selected because those are generally the most well-established faculty members and they know the other program directors from their board meetings, association meetings, etc. The PD doesn't have to write a letter about you, but make sure somebody who is well known, knows you well, and has great things to say about you writes it. I've heard that there are some faculty members/chairs will not write letters for away rotators, and likewise, there are PD's out there who do not strongly consider non-home institution letters. I also agree with what munchkinkitten said about away rotations dictating interviews. Some of these faculty trained together or did fellowships together, and certain letters will carry more weight at some programs. Also on interviews, they'll ask you which programs you rotated at, and you'll need to be prepared to answer why you chose specific programs.
Be aware that at your away rotation at some programs, you'll either be offered the option of interviewing during your away rotation or the away rotation will be your interview. There's also away rotations out there that write composite letters for visiting students - each faculty is offered a chance to add input to your letter, and all the faculty sign it.
