There are nine players at a time on the field for fastpitch softball and ten for slow pitch softball
8 or 9
slow pitch softball requires 10 players, 6 infield and 4 outfield.
13 defensive players
check this out: http://www.sportsknowhow.com/diagrams/softball-slow-pitch-field-dimensions-diagram.pdf
There are two different kinds of softball, fast pitch and slow pitch. Rather than try to explain I will point you to a wonderful link that will explain all. http://www.sportsknowhow.com/softball/dimension/softball-fast-pitch-field-dimensions.html
There is no slow pitch softball for high school girls. In fact there is no slow pitch softball for middle school girls either. I would know since i play for my school.
However many you can until, you pitch out
In softball, there are 9 players on the field at one time. There are also two, sometimes three, umpires. There are also the first base coach and the third base coaches from the offensive team.