The short answer 12-15.

In a perfect world, you’d have 3 pitchers  2 catchers and speed on the bench.  12-15 players is a number that you can manage.  Typically most of the girls will see the field in most games.  You can utilize base runners and the DP/Flex rule to get another girl an at bat.  Often times, you’ll have someone hit for your pitcher (Pitcher=Flex).  This usually works out pretty well, although surely some parent will be pissed at you about something.

Keep in mind that when the season starts, if you have 15, you will more than likely be down to 12-13 after a few weeks.  It’s totally normal for people that want more playing time to leave for a better offer.

As the girls get older (16s and above) it is not uncommon to see teams with up to 20 girls on their roster in the beginning of the season.  Same scenario though, they will most likely trim down to 15-16 after players determine whether or not it’s a fit.  Girls playing in 18G need more players because they will usually depend on a deep pitching staff and base runners for their pitchers and catchers.  They will also have a handful of younger girls eager to earn a starting spot when the seniors leave.

Keep in mind that when the girls get older, some tournaments could easily have them playing 4-7 games on a weekend, especially if you get in the losers bracket.  The girls get tired, so having back up players to sub in can really help you when you need them to cross the finish line. 

It does take a lot to manage more players, but believe me, when you go out there with 10 and one gets hurt or one is sick, you’ll be wishing you had more girls.