I have a RF 87ub 55g and it is currently my favorite keyboard.
I tried the rollover test with my RF 87ub 55g connected via USB, and with various combinations of regular keys being pressed 10 at a time, I get 6 green keys lighting up on the web site test. So, I suppose this indicates 6-key rollover on USB, which seems fairly standard.
BTW, I also happen to be testing a WASD V2 87 mx Brown keyboard at the moment, and when I tried it, I could only get 5 green keys to light up.