It seems to be 6KRO for most key combinations I've tried. Two arrow keys + z + x is no problem.
Other working examples include q+w+e+a+Shift+Space, or a+s+d+e+Shift+Space, or any six numeric digits on the keypad. You can't do z+x+c+Shift+Space however, or even just i+o+p, so the 6KRO mainly seems to apply to common gaming combinations.
Still, for a dirt cheap keyboard, I'm pretty happy with it.