The stash is the most moralised part of making. It is either a charming collection or a symptom of consumption, depending on who is writing, and almost every stash-shaming article is written by someone with a stash.
The number is not the problem
Counting skeins tells you nothing useful. Forty skeins bought deliberately over six years for specific plans is a library. Twelve skeins bought in a panic during a sale, in a weight you do not knit, is clutter — and it is twelve, not forty.
A better question is whether the yarn is still usable. If you cannot see it, if it is not wound, if you have forgotten what it is, then the stash has stopped being a resource and become storage.
Storing it so it stays knittable
Keep yarn where you can see it, out of direct sunlight, and away from anything that will eat it. That means lidded baskets or open bins rather than vacuum bags, which compress the fibre and hide the stash from you as effectively as from moths.
Group by weight rather than by colour. A basket of worsted is immediately useful; a basket of greens of five different weights is a mood board. Keep the labels, or write the weight and yardage on a tag, because you will not remember in three years.
Permission, not penance
The stash exists because buying yarn is part of the hobby, and because a project you cannot start tonight is a project you will not start. There is nothing wrong with that. The only rule worth keeping is that the yarn should be reachable, visible and honest about its own weight.
If you want to reduce it, knit a blanket. It is a genuinely good use for eleven incompatible skeins of worsted, it takes a year, and it will teach you more about colour than any amount of organised shelving.
The only honest inventory
You have exactly as much yarn as you can find, in the weights you actually knit, for projects you would genuinely make. If that is two skeins, good. If it is sixty, also good, provided you can see them from where you sit.