pci express enumeration problem

The BAR themselves (config space) are accessed through special registers.

The memory resources required by the cards end up being allocated from physical memory space.

There are several resources needed for PCI and PCIe cards:

1) IO space

2) pre-fetchable memory

3) non pre-fetchable memory

4) bus numbers (for cards than contain bridges)

A bridge will (usually) only forward a dense range of each to each subsiduary bus.

A reasonable amount of the 64k IO space is available for PCI/PCIe.

Most systems reserve 1GB of physical address space for PCI/PCIe memory.

Somewhere in the windows device mangler you can find what is assigned where.

Finding out about bus numbers is hard!

I'd look at the state with 4 cards, and see how much of what each has got assigned.

Also (if you can) try some cards that have limited resource requirements to see if the whole thing can work at all.