I don't really understand the explanation of a monster's CR…

Do they have one? If so, how many boons does it take to activate it?

I have never been sure about this. I think that as per raw they don't have.

As a GM I have always found the NPC cards too boardgamy. By this I mean that NPCs have a DR and soak/def value but it is not stated which weapon or armour are they carrying. I have always given the NPC "real" weapons like speas, hand weapons, two handed weapons with DR, CR, and qualities. Similarly I give them "real" armours that fit their soak and def values. Although my players are already 36 years old they still like scavenging the corpses.

Cheers,

Yepes

As Yepesnopes said, no they don't as per RAW. However I have noted that several of the creature actions cause criticals, so it could be reasoned that the cards takes care of the "issue". Some creatures just need a hammer or two, or a couple of boons to land a critical hit. My feeling from running a whole bunch of combat encounters with using a lot of the different creatures and their actions is that they crit enough just by using their cards. If I would add a CR to the creatures I fear that my players would rack up criticals way too quickly.

That beeing said, I do give "real" weapons/armour to some creatures, generally important NPC's such as the head of the orc tribe, the evil necromancer, the noble's champion or similar and they will have all the benifits (and drawbacks) of the weapons they are wielding. But for the most creatures I use the stats printed on their creature cards (however I often narrate what kind of weapons/armour they are carrying and the players are free to loot if they wish).

If you just want to assign a CR to "all" creatures in the game you could go with CR 3, most weapons end up there so if you do not want to give all creatures specific weapons and track damage/cr you could just use CR 3.