Last year they interviewed 210 people on-site and offered 89 acceptances (information presented to me at my on-site interview).
They also told me that they have already sent out 310 on-site invitations this year so they will be interviewing more this year.
Personally, I have always considered Mayo my first choice......but I am not holding my breath as I know an acceptance there will be the equivalent of having Ed McMahon knock on my door.
Best of luck,
WISC-ite