Every year, about 10 appointees don't complete I-day. There basics tend to fall into a few catagories:
1) No shows - Appointees who got cold feet between accepting their appointment and arriving for I-day...and never informed USAFA. There are usually one or two of these per year.
2) Appointees who didn't get off the bus - These appointees realized somewhere between the field house/doolittle hall and the footprints that this isn't what they want to do. When the bus cadre say "GET OFF MY BUS!!!", they ride it back to Doolittle Hall and go home with their parents. There are usually one or two in this category per year.
3) Medical turnbacks - Appointees have to pass a medical station before swearing in. If an appointee shows up with a medical issue that would prevent completion of BCT (broken limbs, recent debilitating illnesses, unhealed injuries, etc), then USAFA says "Thanks, but no thanks. Reaply next year." If a Basic Cadet is injured AFTER swearing in (during basic) then they can come back for BCT the next year without reapplying. The majority of 1st day fallouts come from this category.
4) The "Get me out of here" basics - These basics show up on I-day, make it up to the hill to begin inprocessing, and decide to leave before the end of the day. They show up for the wrong reasons and decide within a few hours that they hate yelling and miss mom's cooking. One or two a year.
During BCT itself, quite a few basics decide to start the Form-34 process (outprocessing) within the first few days as a reflexive response to the culture shock. While some proceed with the process and leave, many decide "huh, maybe I can do this" somewhere during the process and decide to stay. However, plenty of basics do leave sometime during 1st or second beast. Most classes are about 60-100 basics smaller by Acceptance Day than they were at the start of BCT.