Common opinion is that there are between 4 and 6 versions available. Microsoft's consumer site indicates four versions – Basic; Home Premium; Business and Ultimate(http://www.microsoft.com/windows/products/windowsvista/editions/choose.mspx), but there's also the Enterprise edition available, so that makes five.
But if you wanted to simply buy a copy of Vista for your existing PC, without going through licensing schemes, how many versions have you to choose from?
A quick look at online retailer dabs.com's site shows 28 versions available.
Some of those versions are upgrade, and 64 or 32 bit, but there are actually only the four consumer versions represented there. It's no wonder people are confused.