Aglages is correct. Prior to day 1 and basic cadet training, it seems that the overwhelming majority of appointees mention wanting to be a pilot and fly. Once they've been at the academy for a year or two; they understand the commitment the air force requires; they realize that there's a 10 year commitment (5 additional years compared to a non-pilot); and they realize the additional academics required after graduating the academy........ many cadets have an epiphany and decide against applying for a pilot slot.
I would say that on average, over the last 7 years or so, it's been around 50% of the class that applies for pilot slots. The good news is: Most years, the academy has unfilled pilot slots and turns many over to ROTC. Meaning, if you're qualified and you pass academically at the academy, you can pretty much get a pilot slot if you want it.
CAVEAT: Receiving a pilot slot, and passing the numerous stages of pilot training and actually becoming a pilot, are 2 totally different things.