For me, I guess it depends mostly on "where I'm going" and "what am I fishing for", plus the "time of year".
Right now, it's three, solid, proven, Steelhead fly patterns. I've had 11 Steelies in hand, this season, already on patterns I've used for years,.so, no use in expanding THAT series!?!!
Trout, if locally being fished, it's 8 patterns, for 99% of the water I hit.
When I head for North Idaho and Western Montana, it's another 4 or 5 patterns, that work better over there than they will here and I leave all but two, of my "home patterns" at home.
So, thanks to this good topic, I'm now wondering "WHY" I always carry at least 9 fly boxes with me, on every trip!?!??
Of course.... I also wonder about a lot of things, I do, that make no sense!?!
Saint Paul-"The Highly Confused"
You cannot do a kindness too soon, for you never know how soon it will be too late.
-- Ralph Waldo Emerson