They just seem to fit so...so...well, in a superhero world. (Which means there's a fair balance between punny villain names like Paul T.R. Geist, and villain names that have no obvious meaning whatsoever, just being their name, and grand monikers.)
Incidentally, the number I currently have? Ten. Totally coincidental, but I love the symmetry and the dynamics present. (It helps that there are at least three on the list which the heroes would be willing to work with on occasion, given the right circumstances, potentially more, if they are desperate.)