Most motherboards wont yield you 6 extra card slots...

Many of the 6 slot 2016/2017 mining boards wont give you any extra, some will give you 1 extra.

I know for example z270 MSI boards give you 2 extra (you lose the onboard video tho).

I think it comes down to the bios and chipset. Just don't expect to expand every board you have.