Having an extra set film holders lets you process more film faster since you don't have to wait for them to dry. (The film holders need to be be dry when loading, the tank/lid can be damp.)
Each film holder holds two sheets of 4x5 film and will work in either the SP-445 or SP-645.
Order a set of two for your SP-445 or a set of three for the SP-645 Max.
Note for those thinking of "upgrading" from an earlier version: if you're happy with your old film holders, there's really no reason to change. Due to the thicker plastic, you'll get away with slightly less liquid in the SP-445, 450 ml.