How much you should stock sometimes depends on your supplier. Some of them have a minimum amount of each item, so even though you might not want a dozen or a gross of a particular style, you'll be stuck with it, if it's the supplier's minimum.Too much or too little is going to have to be self defined. Mike and I often go through this - *I* think we should get 2 gross, he thinks a half gross would be enough. Usually somewhere in the middle is what winds up being optimal.
In Bali or sterling silver, we normally stock at least a dozen of each. Some things we might have more or less of, usually because of a vendor's minimmum on the items. If we're testing the waters, we might buy a half dozen (or the minimum) and see how long it takes to sell them. If nothing else, a vendor's minimum makes you pay attention to what sells. Things like jump rings, french ear wires, earring posts, and lobster claws are the basics that you KNOW you're going to sell, so you might carry 2 or 3 dozen packages of each of them. We have several styles of toggles, but the only ones that steadily sell out are the tiny/dainty ones. The big clunky ones stick around forever. I'm tempted to send the big ones for scrap, but I know if I do, I'll have a dozen customers who HAVE to have something that size!
One rule of store inventory is, no matter what you have and how many you have, it will not be what *they* are looking for. Another one is that you can have a "dog" that sits and gathers dust no matter how many times you reduce the price, and as soon as the last one is FINALLY sold, you'll have 20 people come in on the same day looking for that exact thing! AND they will be upset because you don't have it any more! A final one is to NOT get what you like - pick what you think is UGLY, because it will sell like hotcakes. <LOL> Seriously, getting things you like will usually be more successful than getting things you THINK you should sell! As I said earlier, your store is a reflection or extension of yourself. [Oh Lord, am I in trouble! <LOL>]
Set your prices high enough to reflect the quality (and to give yourself a decent profit), but not so high that you chase buyers off. Some people look at the price first, and then if there isn't quality to back up the numbers on the tag, you'll lose a sale or two. Some people only buy by price. They'll spend $2.50 on a 5 gram tube of Delicas, but think that $4 for a 10 gram of that same bead is too much.
Hope this helps!
Deb