12 is a good roster number. But you I usually think in terms of 4 kinds of minutes: starting minutes, training minutes, backup minutes, scrub minutes. Each of these minutes have different requirements and costs. Group them into players who have the right profiles for each group. Then concentrate on getting the right players the right minutes (48-75) using SFDC for substitutions. And when you have to make decisions to favor one player's game shape over another's it's always clear that the scrubs lose. Then gameshape should be pretty easy to maintain at a competitive level.
A plan like this gives you flexibility to be competitive in two of three games each week, which you can choose based on your cup aspirations and the competition in your league.