One thing I've done before is compare the game's use of players at different positions during a full season to real life usage. So, I'll go to baseball-reference and click on fielding stats for a particular team. Then I'll scroll down and count the number of different players who appeared at each position during the season. Catcher, shortstop, and centerfield, as you would expect, usually have the fewest names listed. First base, second base, and leftfield usually have the most. In my fictional league, I checked a team from last season. Something like 9 or 10 players appeared at SS and 10 players appeared at CF for this particular team.
Here's the fielding screen for the Nationals from 2014:
2014 Washington Nationals Fielding Statistics | Baseball-Reference.com