Here in Malaysia I fish two flies. A Loud-Mouth Popper for Snakehead and a Termite for Gourami. The termite technically counts as two flies because with a black permanent ink marker pen I also turn it into an Ant.
Yes sometimes the fly really matters. We all know the scenario; fish rising for something (probably small and brown) and refusing everything we throw at it until we tie on a fly with the right trigger/image. But mostly it’s nothing like as important as we sometimes think.
I often remember Doug “Hare’s Ear” who was a regular in my Ardleigh days. He would fish two flies and nothing else. One was a short shanked Hare’s Ear nymph and - you guessed it - the other was a long shanked Hare’s Ear and Doug caught fish by the bucket load.
Still when they were eating ants it didn't work!