Less, and yet again more, than I have now! :)
I'm not sure what the perfect amount of stash would be, but the perfect blend of stash would offer me a variety of project choices at any given time, with enough of the chosen yarn for the project.
My stash has many 'not quite enough' yarns as I bought yarns with little clue as to what I wanted to make and/or how much it would take to make SOMETHING, except socks. As a newbie stash enhancer, I bought many orphan skeins or I'd buy two of something, so I might have enough, only to find out that the yardage I need for a given project would really need three.
In the case of sock yarn, I bought it because it was unique or the colors caught my eye. However, if I don't make socks out of a given skein, it is difficult to use for anything else because the yardage works for, well, SOCKS! To use sock yarn for something else, I'd generally need two or more skeins...
So, enough for me would be less than I have now (I'm slowing donating most of the acrylics and novelty yarns), but with a greater quantities of individual styles/colors to keep me going between trips to the yarn shop. And, with gas prices rising and a minimum 60+ mile round trip to just about any yarn shop, the trips will be fewer and farther apart!
Now, if you asked about needles -- the answer might be different! ;)