I don’t know that any college quantifies the percentage. I know that several colleges - Yale and Penn come to mind- have said it’s a low percentage.
My guess is that they only forward those from applicants they are seriously considering, but are on the bubble. As it is not a professor’s main job to evaluate supplements, admissions will likely not waste the professor’s time by sending supplements with no chance of admissipn or who would likely be admitted without the supplement.