When I was working at a small company in Illinois, Vysis, my team had an architect named Ian Poole who taught me many things, and one of them was the simple fact that there are only three numbers in software design: zero, one and every. By this he meant, you can support the case where there are zero copies of an entity, one copy of an entity or an arbitrary number.
I ran into this problem when revamping my iPhone app, Signal GH, to monitor antenna signal strength from an HDHomerun. I had mistakenly written it to handle two tuners in a single device. The code was littered to references to the "yellow" tuner and the "cyan" tuner, as that was the colors of their graphs. Well, Silicon Dust came out with a version with a single tuner, which sort of shot me in the foot. And then I bought that device which gave me three tuners on my network. I had a painful time backing out my bad design choice in favor of a more arbitrary number (I did limit it to keeping track of 8 tuners, but that number is kept in only one place and could be changed in a few minutes.)
The new version will be out shortly, fighting off one last bug.