Crewman cards are not considered players, but are special Characters that are used to reduce the difficulty of a scenario. Regardless of the number of players, before you start the game, you can choose 1 Crewman card.
All crewman cards do not affect the total number of players when considering Shelter, Roof and Palisade costs or in any other case.
There are 6 different Crewman cards available in this set.
For more information, visit the BGG listing.