I was able to detect 6 7850 cards with MSI Z77A-G45 win7 x64 but 2 of the have Code43 error. so it seems as if its a driver/os problem.
Used every slot except the 4th.
If this is the G45... you need to put the monitor on the middle 16x slot/card... that is a "mobo" issue. (Also note, mining on that slot, will send a portion to the first 16x slot too??? Seen if you disable that card in CGminer, it shows as disabled/off but shows as mining about 5-2MHs when the other slot is also mining.) Might be because those slots turn into 8x/8x/4x with all 16x slots used. Thus, shared busses and bad programming of CGminer code.
The limitations comes from the bus hardware, which is why you can only put up to 7 active slots, and only up to 8-gpu cores. (They were not designed to put 8 video cards, they are 16-lanes short of a full 8 slots. Designated for PCI and ON-BOARD crap for motherboards, like network, usb, video, sound, etc. Not an issue for servers, but servers use dual processors, and they split the bus into two halves. Yet can still only take 8-GPU cores in software.)
ALSO NOTE: You can use MAPPING to "discover" unseen hardware in CGminer. But not if windows has disabled it. (That is a catalyst/windows issue, if you can not get them all seen beyond the motherboard thing above.)
Also try adding them one at a time... rebooting and discovering them one at a time.