there gaming keyboards with virtually no rollover and ghosting. They can be kind of pricey though. I can confirm i can press 10 keys at the same time with g710+. Did not try with more, i have 10 fingers.

also this article states the 6 key limitation in USB controllers have long been solved:
http://www.tomshardware.com/reviews/mechanical-keyboard-razer-logitech-gigabyte,3505-6.html

last paragraph.