All I can say is that my boards are stable for weeks at a time running 9 RX 470 4GB cards. I haven't tried 10 cards because my rigs have been so damn stable with their current setup, I didn't want to risk introducing new issues, especially with mining difficulty increases as of late, since the prospect of obtaining additional RX 470s at a reasonable price appears to be minimal at best.

I have a vague recollection of Tytanick mentioning that smOS can handle up to ten cards, but he needs to update scripts before it can handle more, though I may be way off base with my recollection.