Hello!
First, in response to your original question, I'd say 6-8? Not more than that.
Second, keep in mind that those pin backs are not the most reliable.. we only kept traders on our lanyards because the kids had lost one/two in the past. (I can't say for sure if they fell off or were just lost w/kids fooling around with them.)
I think if you keep favorites back at hotel or wherever, it will limit the temptation to trade some of those as well, my kids often got caught up in heat of moment!
Happy trading!!