13 regular and 6 mining-specific if you mix AMD and Nvidia mining cards, per the manual - though I have no clue why you can't do 8 of each.

I am uncertain if any of the BIOS upgrades changed those limits, and have no CLUE why there would be any difference in the interface of a "mining-specific card" that would allow them to be used in higher quantities than "regular" cards.
VERY odd that.