The number of shells you keep in the tank will depend on the species of shell dweller you keep.
In a tank as small as a 10 gallon, I like to keep N. multifasciatus, since you can do a colony of them. I'd start off with about 6. In my tank, I had a sand substrate and completely covered the bottom with shells of various sizes. If you go for a pair of brevis, then you only need one shell, as the male and female share the shell. If you can't find a pair, you should have multiple shells until the pair forms. For species like N. occelatus, you'd want at least one shell per fish. In a 10 gallon tank, I wouldn't attempt a community, although the colony of multies comes close to one. There are a number of sites online that sell shells, sometimes in bags of 50 or more.
Tim