How common ? Likely not very but only one way to find out.
In other words, I’m guessing most, but not the entirety of students at more selective schools come in well prepared. Athletes, first gen, legacy and others may not - or simply kids who are mis judged by admissions.
But what do you define as selective ? Because if it’s what I think, most who are highly rigored and accomplished likely also get turned down.
Take your shot but have a balanced list.
And make no mistake - selective does not = guaranteed life success and not selective does not hinder success.
Only you do…
Good luck.