There actually
are specific rules for groups of Extras in the PDF, actually, and I've collected them below.
You may recall that characters can act together in a Contest, using the best
of their Results (see page 157). This is also possible in a Conflict and it’s recommended
for Extras, whose actions aren’t supposed to frequently turn the tide.
Without surprise, Extras in groups tend to telegraph their actions and rarely
get the drop on Heroes and Villains. When a group of Extras includes characters
with different Wits scores, the group declares based on the lowest Wits among
them.
Some large-scale Conflicts, like the Siege of Luthadel or Vin’s raid on House
Hasting, can quickly bog down if you keep track of every character involved. In
these cases you can keep the pace brisk by letting Heroes and Villains attack
Extra groups instead of individuals. Each successful attack defeats 1 Extra in the
group per point of damage.
CROWDS
Mental Conflicts occasionally target multiple characters at once. When this
happens each target rolls and the best of their results is used — just like any
group of characters working together (see page 157). Beyond a few characters
this becomes quite cumbersome, however, and in these cases we recommend
rolling once for everyone, awarding a free Nudge for every 10 characters in the
crowd (maximum 5 free Nudges). This should only be done with Extras, of course
— Heroes and Villains are far too important for this treatment.