When it comes to leadership...it matters more about quality than quantity. Certainly, a few leadership positions are great, but too many can indicate that you are not really focused on any one activity. This is one area that USNA Admissions relies on the BGO to help amplify through the candidate interview. You should expect to be asked what you do in your leadership positions.
Edit: Also, do not think you have to be in a leadership positions in EVERY activity. It is certainly acceptable to be "just" a participant in some.
The Admissions process is focused on the whole person...to say it simply, you have to be good in many areas (academics, athletics, leadership, activities, time management, etc.), but not necessarily "perfect." In the military we sometimes refer to this as a "mile wide, inch deep." I wouldn't worry so much about the percentages, stated above as strict figures (although -- academics is typically a very important piece), since the Admissions Board can change the candidate multiple and their decision to give the nod on the Admissions Board qualification might not directly be tied to the multiple.