I think I've seen more than 6 out at a time. I think the minimum is 6, because I've seen them being replaced when it dips below that number.

I really think its tacky for a freont desk person to limit who gets the bags, even if it is to keep down costs. How much does the bag cost? $1.50 to make?

I've been to HI's where if they know I'm leaving really early in the morning, they'll grab me a bag when checking in so I have something to eat in the morning. Really nice touch.