If you make a Stealth check (opposed by Observation) and there are 10 guards, do you take the best Observation (of the guards) and add some misfortune dice for the extra number of opponents?

Sounds perfectly reasonable to me and far easier than rolling 10 separate checks.

I think that's the best method. for up to 3 or 4 guards I would roll separate (the new dice pool system is so much fun when you got used to it).

Ok thank you :-)

I think the core mechanic and the asymmetrical system encourages the idea that it is the PC against the world. I think generally if you can make a single roll for a challenge then that is a good thing. It also works narratively. No one wants to know if the guy sneaks past the first guard then sneaks past the second guard then sneaks past the third guard, etc, you just want to know if he sneaks past the guards (if they're all in the same place). So I definitely try to keep that sort of stuff to a single roll, and, more importantly, a single roll for the PC, not the guards. I'd be inclined to simply add one (or two if they're trained) black dice to the PC's pool for every extra antagonist.

In a more simulationnist vein, I would take the best observation skill, then add as many Fortune dice as there are other guards actively helping. Typically, in any group, people tend to rely on one another. We can imagine that in any group of guards, a few of them will be the more vigilant ones and the others will rely on them for their safety. Sometimes, though, everyone will rely on someone else and ... no one actually gets the job done !!

So in the end, the idea is similar to what others have suggested. It's more about how vigilant the group is as a whole. And as I suggest, a group should not be more vigilant than what 3 or 4 of its numbers can be, while being active and working together. That would be the top level of awareness, ie. the guards know they are in a dangerous part of town.

In the case where the guards are off duty, drunk or playing dice, I would not make this roll opposed. I would just set the level of difficulty based on terrain, illumination, the guards condition, distance to the guards (ie. Easy would be good) and have the player roll.

That's the way I adjudicate observation for the group also. I ask them to design who's on watch, with the possibility of having one or two people assisting. If they all want to observe, then I'll ask for rolls only to the ones who are the closest to the stimuli (or ask rolls all around but take into acount only the pertinent ones).

The end point here is that a group of people isn't as effective than the sum of its individuals for observation. For one thing, the group distracts itself by its actions.

I think a fun way of doing it is basic challenge based on opposition by the best opposing ability, and say "that's typical for 1-2 guards, then the other 8, well maybe they are all watching and making it tougher or talking among themselves making it easier, so that's 2 black misfortune dice and 2 white fortune dice both".

Like the intoxicated condition, we don't see if there being a number of them hurts or helps this time until the dice tell us. Part of what I like about the dice system is you don't add/subtract bonus/penalty before the roll, you add dice to see how they worked out, and then cancel out boon/bane etc.

Rob