I usually like to have between 23 to 25 players, 2 for each position and 2-3 players that are young and/or are versatile. Then, i try to rotate against lesser teams and when we have a congested fixture. Even when everybody is fully fit, i pay attention to risk of injury and how many matches they've been playing in the last 3/4 weeks. That way i don't overotate in a way that my players can't form relationships, but in a way almost everybody can play.
I try not to change too much of my first eleven when i rotate, 3/4 players are enough unless we are in a run of really important matches (Imagine February/March when playing against the big teams of your league, while champions knockouts and cup games) where in the middle you have a match agains a middle/low table club, in that game i would change 7/9 players of the starting 11.
Which ones i rotate comes down to low Natural Fitness and Stamina and of course how intense is my tactic.
