just for fun I looked up Iowa's playoff system.
the four smaller classes (8-man, A, 1A & 2A) each have 32 teams go to the playoffs. (they play 8-game regular seasons)
the 3 larger classes (3A, 4A & 5A) each have 16 teams go to the playoffs. (play 9-game regular seasons)
four teams in each class go to the State Semi-Finals.
IA uses a 'district' system, and the number of districts vary by class. In the smaller classes, 4 teams from each district advance. in other districts, 3 teams advance and then there are 'wild-card' teams that advance to even out the brackets. 5A does its own thing (like 6A in MN).
so, in general, in most districts, it looks like roughly half of the teams make the playoffs.