As a rough guide, the cheaper the lock, the more likely it is that your neighbour has a key to fit it. In the case of door locks, a 2-lever lock (the type that the builders fit to keep their costs down) may have only a dozen differs. There is a British Standard for door locks and to meet the standard, there must be at least 750 differs. Many manufacturers go further and provide over 1000. As BillyBB says, there are also locks, usually for high security applications, that more than meet these standards.