One thing interesting in this screenshot is the super low PCIe bus number 3 for the slot. The M1 Macs don't use many PCIe devices. Most of the devices are integrated in the M1.
In fact, the M1 Macs have three PCIe devices at address 0:0:0 meaning they are all on separate PCIe buses (segments? or PCIe trees):
1) Bluetooth/Ethernet
2) Thunderbolt bus 1 with 128 PCIe buses (why not 255 PCIe buses? if it were limited to 7 bits then the last bus number would be 127 instead of 128)
3) Thunderbolt bus 2 with 128 PCIe buses
No other PCIe devices exist in M1 Macs except what you connect to Thunderbolt. The M1 Macs have 128 PCIe bus numbers for each Thunderbolt bus. That's the most of any Mac because most Macs only have one PCIe tree and many bus numbers are used up by builtin PCIe devices. The M1 Macs have a separate PCIe tree for each Thunderbolt port. A Thunderbolt device takes between 3 and 7 PCIe bus numbers. Besides PCIe bus numbers, there are other PCIe related resources required for each PCIe device which may limit the number of devices allowed.
It would take about 21 or 22 Thunderbolt docks (e.g. the OWC Thunderbolt 3 dock uses 6 bus numbers: upstream, downstream, two USB controllers, FireWire, Ethernet) to use all the PCIe bus numbers but you can only have 6 deep for Thunderbolt (5 for USB4 but the USB4 registers have room for 7?). Therefore you would need to use Thunderbolt 4 docks with multiple downstream buses. OWC has one such
Thunderbolt 4 dock. It has 3 downstream Thunderbolt ports. However it could be using only USB devices so it might only be using 3 PCIe bus numbers (upstream, downstream, USB controller). In that case, it would take 43 of them (or the smaller
OWC Thunderbolt Hub which also has just USB with three downstream Thunderbolt ports). Or 8 of them and 15 regular Thunderbolt docks (at the end of a chain, add one bus number for the empty Thunderbolt port, which makes 7 bus numbers for the OWC Thunderbolt 3 dock).
