My understanding is that if you use VGA, DVI, or HDMI, you can have up to three monitors. It has to do with how those outputs work, the limit is three. For more than that, at least one output HAS to be display port. In addition, it's possible to daisy chain display port connections, which is why you see more than four monitors being used by some people.
That's why you won't see 6 HDMI monitors running off one card for example.