I've also been unable to get more than 2xPro Duos working properly in Windows 10.

Problem seems to be be in BIOS which can't allocate enough resources with "Above 4Gb decoding" disabled.
Enabling this option seems to cause all sorts of problems with the Pro Duos, including making BIOS inaccessible thereafter (requiring CMOS reset).

Without it, best I have managed is 5x Fiji with 3 Pro-Duos' installed (wasting half a card) or 4 x Fiji + a RX470 (which can't use same drivers so no=go)
I once got 6 cards recognised briefly by disabling the USB ports but then couldn't use a keyboard/mouse. :-)

I've tried with both X99 and H270 mobos. Hoping for better luck with Z170 Gaming Pro Carbon but I hear that Fijis are a problem there too.