I was speaking off the top of my head, hence the ~ preceding the 5. That said, more clusters may be visible on a 3D graph. The data itself has many more dimensions, and we see only a projection.
Perhaps more dimensions would reveal five or so clear clusters that correspond exactly to the races listed in the US census, but I suspect what it would actually reveal is that population genetics is really complicated and has lots of counter-intuitive cases that defy most people's naive expectations.