Hardware Passthrough

I have gained some interest in hardware passthrough. I have been reading up on how the vfio-drivers work and was a good way to pick a motherboard. Really what I would like to know is, what factors determine the number of Iommu groups and which components are part of each group. Is the only way to actually test each board and find out after buying? I am curious because on my current board, I would do a passthrough but, I have a pcie nvme which I want to use for both my main system and my vm. When I set up a gpu passthrough I was getting both components as part of the same group. I couldn’t get the acs patch to work but, I think that was due to a lack of research at the time of my attempt.

I am hopefully going to build a new pc next summer using threadripper if others see that it is a good option for passthrough. Is it a good assumption that, given more lanes, there maybe better iommu group support?

Also. are the groupings based on the number of iommus on a board (or is there only one) or based on bios( where better isolation can come from bios updates)?

These may be stupid or already answered questions but, after searching for a while I didn’t really find these answers. It seems most questions are about getting it to work as opposed to the actual architecture or mechanism by which it works.(correct me if I am wrong or just really suck at looking for answers.)