You need at least 4 RB's and that is usually the norm. That actually is the way to go there.
Running backs (4)
With the NFL becoming a passing league, most teams still keep four backs on the 53-man roster due to injury history at the position. Most NFL teams employ a backfield by committee approach and
most teams include a fullback in this number.
Special teams play a huge role in this number as well. Most running backs have speed and can be a force on special teams.
One of the best in the NFL is Jacksonville Jaguars running back Montell Owens. Though he is listed as a fullback now that Greg Jones has departed, Owens was seen as a special teams demon and Pro Bowl player. This increases his value and keeps him active on game day.
If a team like Jacksonville dresses a fullback for the game, it will probably dress three other running backs as well. The starter, the primary backup and a special teams player should all dress on game day.