for fruit/fish/bugs/etc. i can just tell you roughly the exact amounts of what i have at any given moment. i have a capacity of 575, and i don't think i've ever bought extra inventory storage, but the closest i usually get to being full these days is 560 or so anyway.
x10 of each basic fish/bug
x10 red snappers, horned dynastids and black bass
x25-30 of each regular fruit (including coconuts)
x15-20 of each perfect fruit (not including lychess, which is my special)
x10 of each shell
x30-40 pearl oysters (although i'm only on 25 at the moment)
and then however many of each expensive fish/bug you can carry after. like, right now i have x15 tuna and x16 sharks, a couple of giant fish, and then some more basic fish. (coelacanths, koi, soft-shelled turtles.) i have way less bugs though. only x10 stags and a couple goliath beetles. but, yeah, that's how i manage it. i don't keep anything that's out of season or event exclusive either.
i've never actually maxed out my clothing/furniture storage though despite playing since launch. i know i'm missing some of the marketplace items and haven't crafted anything i don't like, but i do have plenty of duplicates for some things. (i'm crafting duplicate mazes pieces right now, for example.) i suppose it's just about how much you keep. i don't change my campsite layout regularly, so i don't feel the need to have multiples of most furniture, and i give all my fortune cookie furniture/clothing duplicates to gulliver.