Hello, muttly! Ditto what splitrock said. Your city/county ordinances will tell you whether or not you are allowed bees, and how many hives (usually by the amount of space you have). Not that we want to add another burdon to an already faltering relationship, but is your neighbor a gardener? My guess is he may not say it, but he'll like reaping the benefits of your foragers!
I also recommend two hives because it gives you a basis for comparison and another set of resources. If you really want to start with just one, though, just be sure to get active in your local club and get to know other beekeepers around you. Sometimes you need an additional frame of brood or a frame or two of drawn comb (and usually in a hurry), and it will be nice to have those relationships in place. Beekeepers love to help each other out!
And, of course, there are always folks willing to provide input here!