The optimal number of keepers depends on how serious and dedicated your owners are. If all of you are hardcore football guys, the more keepers the better. Keeping most of your players would allow someone to build a dyasty and act as a real GM.
Personally, I like to have three keepers, it gives teams a chance to build for the future and become a dynasty, but it isn't too many keepers to the point where bad teams will just want to quit altogether.
I am currenty involved in a 12 team, 3 keeper league. We have added a twist to it. We name one of our keepers a franchise player while the other two keepers are transitional players. The franchise player can remain on your team forever if you choose so. With the transitional players, we keep them on our team for three seasons, then they will go back to the draft pool.