Personally, I would lean towards 6 flies per clam shell if those are the only choices (4,6,12) Those things (clam shells) are expensive.
I would think you could go with 25 for the beads etc as plastic bags are only a few pennies each and I like to break up small parts when using them. ( If my kids get in a bag of 25 instead of 100 I'm much happier)
just my 2 cents
good luck!