Last year my daughter's team made it to regionals. They used one pitcher the majority of the time, but had a couple others that picked up a few innings here and there. I would guess the #1 pitcher threw 90% or better of the total innings played.
If you have a dominant pitcher as has been said they're gonna get the large majority of the innings. I would guess most teams sans injury aren't gonna go more than 2 deep.
Pitching is an area all schools try to develop as many ladies as they can but realistically for most schools they have one that gets the majority of innings and a second that may start a handful of games vs. weaker competition..
If they are lucky teams may have one lady on JV that may be able to get varsity innings and a position player that can throw a few innings.