30 pairs of shoes? 17 Star Trek bobble-heads? When you're trying to unclutter your world, it would be so useful to have guidelines for the ideal number of socks, or books, or teacups. I appreciate minimalism, but I don't think it's right for everyone, and I don't think the goal should be to live a monastic life. Unless you're into that, in which case get rid of everything, today. Boom, done.
One thing I'd advise against when decluttering is to get rid of things just for the sake of having less. The idea is to toss the excess, not the useful. So if you happen to love repainting your nails to match your outfit, 20 bottles of nail polish may be just the right number for you. If you are training for a marathon it might make sense to have 10 pairs of running shoes. And if you are a fiber artist you might actually use all of the 9 pairs of scissors you own. (I'm looking in the mirror here.)
There is this one little word that really makes it hard to figure out how much is enough; that word is NEED. How many pairs of socks does one really need? I could likely get by on two, if I were willing to wash one out every single night. But that doesn't seem like a good use of my time and energy. How many houseplants does one need? Well, none, really, but they sure brighten up the place.
It makes a lot more sense to stop asking what you need and start thinking about what you use. Do 10 of those 20 bottles of nail polish merely sit on the shelf collecting dust? Is there one pair of scissors that you never take out of the drawer? (Ahem, staring pointedly into the mirror this time.)
We only need a fraction of our possessions for survival, but life is enriched by some of those unnecessary things. So you could expand the word "useful" to encompass things that brings you enjoyment and ease. Clearing out is supposed to make you feel lighter and more free; if you're creating a sense of scarcity and self-denial you are going too far. Keep what you need, and keep what you use, and keep those things that bring you happiness. That is the exact right number for you.