Tom,
At the very top of the form you'll find that it specifically states that the reference can not be a relative. Otherwise you can use pretty much anyone. However, I have heard incidents of people having their references rejected because their reference did not have a clean background. So try to be selective of who you choose. Likewise, while it doesn't state anything to that effect, it seems to me that the general consensus is that it has to be someone who has known you for at least 5 years. As far as more than one per household (i.e. perhaps a friend and his wife) I'm sure that wouldn't be a problem, though it would probably look better if they weren't.
