I’ll answer these both questions together.
What would you consider the perfect amount of stash?
I think, there is no perfect stash…
At the one hand I would have all sock yarns, on the other hand a big stash can crush me and during all the things I don’t fell well. At time I will reduce many things in my household (books,…) and only buy the things, I really need.
What yarn (that you don’t have/haven’t used) would make your stash “complete”?
I think, there’s no chance to make my stash complete, because there are so many nice yarns I would like to try. But of course there are yarns that I’d like to try, f.e. Blue Moon Fiber Arts, Flat Feet Sock Yarn, Wollmeise,….
What yarn do you never want to be without?
Definitely sock yarn.