USB controllers are usually unable to provide more than 6-key rollover. Some keyboards offer two parallel cables for 12-key RO, but for real NKRO you basically need PS/2.
I wouldn't say "usually" anymore. I was just able to mash 18 keys at once on my non-exotic keyboard and have them independently register. NKRO over USB isn't that rare now.
Maybe that was the case in the past, but I don't think that's true anymore.
Using this[0] page, it will register as many keys as I can mash without triggering a global hotkey.