7. Minus the Xfactor mess, she's done a great job with her performances this era. Her voice might not be powerful, but its great at expressing emotion, which I'll take over empty belting any day.
Rihanna's voice is really bad. I mean really bad, especially for someone of her caliber. She's usually flat, no belting power, very little range, not creative vocally, etc. I know her fans cant hear it, but she is not a good singer.
You can't compare them to Beyonce, she would slaughter them on stage, vocally or otherwise.