That has to be one of the most open ended questions you could ask. It's dependant on so many things, the water, it's type and location, the time of year and day, what is hatching, so many small variables. Some days one fly just seems to catch and others your swapping every half hour. My favourite for sport is either a team of buzzers or a single fly, great takes and usually a better quality of fish is your at a still water, than you would get with a lure.