So it sounds like we should expect to hear something by April 15 if we are getting in, and if not, expect a TWE on the 21?
Another possibility would be hearing nothing by either of these dates, which would indicate a candidate is very much on the bubble. There will also be those who receive a TWE and could later receive an appointment offer anytime up until around mid-June a couple of days before I-Day.
For those who have received offers for USAFA, USAFAPS and FFS and don't accept by 4/15, it gives admissions a clearer picture of how many additional offers can be made to each of those programs.
Think of the numbers admissions typically works with and they've been doing this a long time, despite implementing a new information system this year. From approximately 11000 applications there will yield approx 8000 candidates, of which they will select a class of ~1100 USAFA cadets, ~250-300 for USAFAPS, and ~60-100 for FFS. On Apr 21, they're pretty sure of the 5500 or so who wont be selected, but it's possible there may be a few offers made to some of those candidates after Apr 21.
There will definitely be a pool of I'm guessing 100-150 who won't hear anything when TWEs drop. If you're one of those, you'll still be in the running, and you definitely want to be prepared with a plan B,C and D.
My DS was on the bubble between the TWE drop for mid-April of 2020 until Mother's Day when he received an FFS offer.
If USAFA is where you want to be and you don't get selected, remember that TWE only means you didn't get selected THIS time. You have several chances to reapply since candidates must be between the age of 17 and 23. My DS has two classmates who are brothers, one is 22, the other is 23, both are prior enlisted Air Force. Selection doesn't always come on the first application.
There was a statistic shared by senior cadet when DS and I toured USAFA in 2019 that 30% of the incoming class are reapplicants. So, I guess I went the long way to say it's possible some won't hear anything by April 21st, they'll still be in the running until a TWE or an offer arrives. For those who receive a TWE, make sure your plans B,C,D are working to fruition and you can strengthen your application and re-apply.
Best of luck to all, it's a difficult wait, but something my Dad once told me is "nothing easy is worthwhile".