"Bench" players are not really a roster spot, but rather a definition related to "starting lineup" rules, as per your league management service.
Roster rules are decided by the minimum and maximum your roster rules allow for selecting at any one position.
For example, in ESPN you may look at your Roster Settings and see something like this:
You should ignore the "Roster Preview", as that is related to starting lineup.
When setting your "Roster Positions" in FanDraft, you would consider the MAXIMUMS you have set in your rules, as well as MINIMUMS as decided by your league.