Windows 10 limitation is 8 amd + 8 nvidia cards, which means 8 per driver. So I guess with this board you can put 8 amd and 4 nvidia + use igpu to control them or vicer versa, 8 nvidia + 4 amd either with or without discrete onboard gpu on cpu.
powering only mobo molex and not pcie risers
What do you mean here? If you mean powering pcie risers, that depends on the gpu you have, if it does not need then no need, for example gtx 1050 dont need any power from the psu as 6 or 8 pin power, so you need to use only the molex, the molex will give 75 watts.