I think the rule is that a pitcher can have his real life number of starts plus 5, with the exception that any pitcher can start up to 20 games.
So, pitchers with 0 to 15 real life starts can all start up to 20 games.
A guy with 16 could start up to 21 games.
A guy with 20 could start up to 25 games.
A guy with 33 could start up to 38, and so on.