NPC crews will be a generic invisible NPC type that you will be able to buy from certain facilities, they are not like current visible NPCs that can follow you around.
A JM5000 needs a crew of 4??? while an S40K only needs 2 or even an YT-2400 needs only 1 (even with 8 heavy lasers).
There are more examples, just check out the technology ships list.
It is also likely based on the canon sources for those ships which generally say how many people it takes to crew them, as well as how many people it can take in total.