The maximum number of gpus supported on one board also depends on the model of gpus you are running.
Since you can only see 4 cards in device manager, it might be a bios issue and not easy to fix.
I would suggest you try the method of shorting pins of A1 b17 and see if it would get more gpus to work.
as per title. i got MSI 990FXA-GD65. i connected 1 master card on x16 riser and other 5 on x1 risers. i did connect 6pin power onto motherboard. power is fine. all the guides mention catalyst 13.1 from techpowerup site or 12.6 from amd website make 6gpu rig happen - though none of them support new r9 290 cards. im now on 13.11 beta 9. device manager only see 4 video adapters. please advise.
http://pl.msi.com/pic/product/five_pictures1_2401_20110531112823.jpg