When my oldest recently transitioned to High School, I thought I'd be shifting from my 12-batter little league over to my 9-batter pro book to cover his games.
I've gone back to 12.
High School definately follows the substitution rules more closely than little league, but the extra boxes are handy for things like designated hitters, extra hitters, and extra diamonds for defensive substitutions which are still more frequent (by a lot) than pro ball.
Going back to "extra hitters".. I hadn't heard of such a thing, but many coaches will, for example, bat 12 but field 9. Substitutions are still more deliberate on defense, but if he only has 12 kids, it starts to look like little league again. If he has 16 kids, he can sub his other kids into more spots. I've seen a DH with an arm injury bat while another kid plays defense, etc.
Bottom line is I thought that 9-batter was going to be for all 90-foot ball, but I actually am very accostomed to the flexibility of 12-batter books, and will be getting another one soon.