I do not have a ton of flies, but I like to categorize my flies by the hatch. So I live in W. MT which means I need a box for my stoneflies, and I buy the 4 compartment pretty big compartments (4"x6") and I fill it with the dry and nymphs of that stone fly, so I only need one at a time. I have mayfly hatches sorted by dries (parachutes, cripples, standards, comparaduns, and CDCs), and a box of mayfly nymphs that cover 8-18. It is a slim mylan box, I can about 20 bugs per compartment in the bigger bugs sizes there. And then I carry a 3 compartment dewitt box with my streamers. So anytime I go fishing I have a nymph box, stonefly box (if the hatch is on), streamer box, and dry fly boxes of the hatches that are happening. I have been caught when an unexpected hatch occurs, but... again, my advice is worth what you paid for, free.