Editing Unwritten: System Reference Document/Running the Game (section)

=====Nameless NPCs as Obstacles=====

The easiest way to handle nameless NPCs is simply to treat them as obstacles: Give a difficulty for the PC to overcome whatever threat the NPC presents, and just do it in one roll. You don’t even have to write anything down, just set a difficulty according to the guidelines in this chapter, and assume that the PC gets past on a successful roll.

If the situation is more complicated than that, make it a challenge instead. This trick is useful when you want a group of nameless NPCs more as a feature of the scene than as individuals.