The optimal bench depends upon your goals.
If you are going for a title, then having a more productive players available makes sense.
In general, if you want to be able to manage gameshape you need 10 players with each player having a single backup.
Ideally, in that scenario, you would want your starters to be worth 3X more than your backups. This is because your starters are playing 36 minutes vs 12 minutes for backups.
If you go with a shorter rotation such as a 8 man rotation, you will run the risk of injuries causing problems, and cup games will be more difficult to manage. To me this is ideal if you are not trying to go for a title and are simply trying to have a decent team. Obviously, you can save money in this method.
Going with a bare-bones lineup of 5-7 might make sense if you do not need to win games and have a solid position in the league.