Generally you pay more for sandpaper in smaller quantities. I know WalMart sells 3M sandpaper packs in various grits. Home Depot sells Norton, which I don't think it is quite as good as the 3M. Buying
150 and 220 grit in 50-count packs would make sense--these grits are most often used. I keep extra papers weighted down to prevent the corner from curling--other than that, sandpaper doesn't have a shelf life. I like the cloth-backed sandpaper which is a red-pink color, more expensive, but it lasts 2 or 3 times longer than the paper type.