Welcome to the world of mining and BIOS modding.
what happens if i need to install the card on another rig? Will the card be not recognised and then i d need to uninstall drivers with ddu and run patcher again?
You install it normally. Plug it in and run windows OS. It doesn't matter if the windows OS is not yet patched with the pixel patcher. It will display error code 43 but you will still be able to run windows and view your screen normally.
If i have 6 cards already modded can i install the 6 alltogerher and run the patcher? Or i need to do it one card at a time with only one card on the motherboard?
It is always best to do it one card at a time. Basically, install GPU, turn on windows, let windows install the driver, shutdown computer, repeat.