Windows 10 dont have a 8 card max limit , you have to use patch for amd cards only if you modded your bios .
so one more times ... you dont need to do anything special to put together a 12 cards mining rigs !
With factory bios on amd cards just install your cards , latest driver (set to Compute), and you are ready to go
same thing with nvidia ... You dont need any trick ... But you need your Brain

I didn't say it is Windows 10, did I?
Are you sure all of the people here know what and how "set to Compute" is? I know it and do that is called as a trick.
If your rig is not stable[/i] its mean you overclocked / undervolted your cards way to much......
Down time ? If your gpu not hashing , its mean that gpu not working , check the cooling fan if its stopped , thats the bad card , turn of you 21 cards rig , remove your non working cards , and you ready to go with 20 cards ...
That is why You always have to fine tune each cards separately

There is a option in afterburner for that , or you can use OverDrivenTool to set up frequency / voltage for each cards

Honestly, this one's really helpful. Will be a good input for me.