I will let more knowledgeable folks chime in on how many the NA weeds out, but there are a lot of applicants who simply take themselves out of the mix by never completing an application or stopping well short of finishing.
There is a lot of controversy of this... Google it!! Many colleges do not agree with the NA stats that they are one of the most selective colleges because they claim they have 12,000+ applicants, but only 1200 are admitted. This is just false. NA counts every student who even applies to NASS, and who even STARTS an application (not even finishing) as part of the 12,000. In particular, Google Prof. Bruce Fleming and his research. He is a current professor at NA and has some very interesting insight into the whole process... He is one who is not afraid to speak his mind...EEK!