It's actually always a good idea to carry a few extra players on your minor league teams just in case of injuries and callups (especially in the upper minors). Usually Class A ball has more players on their rosters too due to development, there is a thread on this down below that will document this and also has a link to the roster rules from MLB and MiLB. Check out that thread and you'll see what I'm talking about. Here is the link:
http://www.ootpdevelopments.com/boar...ter-limit.html