Well, the perfect amount of stash eh?
There should be enough for you to be inspired by it, but not so much that you’re overwhelmed by it.
Ooh Pretty not Oh my God I have more Yarn than I could ever possibly knit!
There should be enough to keep you going for at least three months, in case you lose your job (a bit like savings in the bank)
There should be enough that non-knitters can’t figure out why you need all that yarn but not so much that your OH wonders why you need all that yarn!
There should be enough that you need additional storage for it (otherwise it’s not a stash)