With good presentation, you can imitate almost anything with only a few basic variations in:

1. Shape: mayfly nymph, mayfly emerger, mayfly dun, mayfly spinner, caddis pupa, caddis adult.

2: Purpose: Floating or sunk.

3: Size: 12 through 18 or 20 covers nearly all situations.

4: Shade: all you need is dark and light.

In the old days in the Catskills, there were guys who carried not much more than Light and Dark Cahills or Light and Dark Hendricksons in a variety of sizes in wet or dry to match most of the prolific mayfly hatches there. You don't need to be quite that ascetic, but it shows that you can get by with a small basic selection most of the time. (A modern update to this approach might be Adams or Hares Ear Parachutes (for a dark dry/emerger), Light Cahill or Sulphur or PED parachutes (light dry/emerger), and GRHE (light, fat) and Pheasant Tail (dark, slender) nymphs in #12-18. Toss in some light and dark Elk Hair Caddis in 12-18 and you're laughing.) Supplement the basics with a few specialized patterns that are proven for the time and conditions where you'll actually be fishing, and you'll be ready for anything.