Eight to ten dozen back up
Are you sure that's enough?
My rain jacket has two pockets capable of holding the largest fly boxes (outside of those bugger boxes) made, plus an inside pocket for a sink tip wallet, when I carry it. I used to carry those large fly boxes plus two water bottles in a hip pack....Used to.
I am not a pack mule. I don't want to be carrying any more than absolutely necessary when I'm waist deep in moving water! I don't want to be top heavy. And if I have to change a head or sink tip, it sure ain't gonna happen out there in mid river.
Now days all of the stuff, and more, is carried in a large Loon Outdoors boat bag. That bag is loaded in the car or boat, whenever I go out. If I'm on foot, I load up one medium size fly box with whatever flies conditions dictate,
maybe add a couple different sink tips to the tippet material in my shirt pocket, stuff the fly box & a water bottle in my waders & I'm good to go. Fish a run, back to the car/boat, move & fish another run. If I take the boat out, I can load more stuff in the boat, maybe two rods, lunch & drinks in a small cooler.
Works for me.