There was a time years ago I thought you needed a bunch of different flies in a bunch of different sizes. So much so I spent what would be a small fortune in hooks feathers furs tinsel, boxes to keep materials in, and numerous fly boxes to keep the myriad of flies I'll never use in my vest. "Just in case" i think I now know where/ why that saying was coined. Because thats where the flies stayed. In the case.
Eventually I figured out I spend the majority of my time >85% on 4 streams. Those 4 streams are all within a 50 mile radius and hold essentially the same flies. The other 15% of the time i may fish other streams but never feel out gunned for lack of suitable options.
I also realized that my fishing is from ~ mid April to ~late June. The number of different possible flies I see that are fishable is 8 total Mayflies, caddis, and stoneflies. I add 2 attractor patterns because they work. Adams and Royal Wulff. In total that is 10 flies with 2 sizes each except for BWOs which is 3.
My nymph box similarly has few options. Mostly 14s and 16s. Some generic ie PN,PT, and HE. A few attractor type Frenchie, Rainbow Warrior, Green Weinie, and 3 perdigons of my design in green, brown, and grey.
That's it. I dont streamer fish so I dont tie them or buggers. I also dont see fish so selective on terrestrials that I can't convince them with another option. If one is that selective, I take the "L" and find another fish.
Keeping it simple keeps my head in the game and not chasing my tail for the latest and greatest fly. Saves time and money and in my mind makes me a better fisherman.