In softball, how many players are on the field when the team is playing defense?

Bad question, very common for teams to use 10 players on defense.

Slow pitch softball has 10 players. Fast pitch softball has 9 players. The question was not well presented.

our leagues always had 10 players

Wrong. Many leagues use 10 defenders, with four outfielders (a "rover' or short fielder in right center or left center.

Would have been nice to set out in the explanation what the nine positions are…