I'd say you're right on with the 1/4 pound per sammie estimate. But you might want to figure 1/3 pound finished pulled chicken per person just to be safe.
I've never done "pulled chicken" before so I can't help you with how many birds to cook. I always do parts. Mostly legs & thighs.
If it's gonna be mostly girls, that 1/4 might be plenty. But if there are going to be guys... Well, you know...
Roaring Fork BBQ :-D
Modified Traeger 075