Only about5-10% of candidates get an LOA, so it is very small (200 -300).
I doubt there is anybody who could tell you how many statistically don't accept it, but also LOA's convert over to an apptmt when you get that nom. Thus, I will go with the guess about 400 candidates out of those that get an apptmt turn them down. My guess for this is that typically 1600 apptmts go out and 1200 show up at BCT. If you are using this as a "well if X candidate turns it down, I might get it" realize that the SA's have been known to call up a few days before BCT and give out apptmts than, so if that is the basis of your question the answer is your roller coaster ride may not end until next June.