If I know what flies work, I usually take or try to take 6, but tht is to cover whom ever I am fishing with.
On stillwater I generally don't loose flies but it has happened one or two times and it is always the fly I have ONE.
River a little different story, it is easier to loose flies on rivers with brush/trees or fighting the fish in the current and a light tippet.
I can say, On my Pontoon or Navi. I do pack all flies, but rivers I take one small box with nymphs, one with dries, one with chironomids or pupas and one with woolly's or sculpins.
Up on the creek, one box, filled with WD40's (always size 22) a few Royal Wulffs and a few BWO's generally in 18's and 20's.
WINTER TIME Everything in 26 and smaller.
Some things are better left alone, Even though, some feel they know better.