I had to track it in UGA games, and we averaged just short of 4 between the two teams.
I tracked our HS games last year, and it was between 1 and 2 per game. Thankfully, I don't think any of those were our kids. Helmet fitting is something about which we are diligent, to the point of the kids being uncomfortable. I got mad at the kids at special teams practice one day, for them not being ready to run on the field. I made every player put on and buckle their helmet and not take it off for 40 minutes. Even the trainers told me not to do that again, I was causing too many headaches.
Helmets should require two hands to remove, by grabbing the edge near the jaw and pulling outward. And yes, it should be uncomfortably tight. The days of putting it on when you leave the locker room and keeping it on all of practice are gone. If you can do that, it's not tight enough.
But I can't stand seeing college players picking up their helmets and simply dropping it back on their heads, flipping it on with one hand. That's a concussion just waiting to happen.