Okay.. guess I have to reiiterate what I said some pages ago:
Yes current ryzen cpu have 24 pcie 3.0 lanes. Of those,
16 is reserved for dgpu. X370 enables CF/SLI by splitting these into 2 x8 lanes.
4 is reserved for communication to chipset.
4 is free. This is why all boards have 1 m2 nvme slot, easy wiring I guess. So on all current boards the first m2 slot for nvme drive is handled directly by cpu.
Now all chipset adds incremental (by 2) pcie 2.0 lanes and general purpose i/o like sata, sata-e and usb.
Starting from A320, it adds 4x pcie 2.0. It does not allow overclocking.
B350 adds 2, so that makes it 6x pcie 2.0 - used for wifi (x2 I presume) and in this particular asus rog strix b350-i board, adds 1 m2 slot running at 4x pcie 2.0
X370 adds another 2, while also enables multi-gpu.
I learnt this when researching how asrock board can have 6 sata ports when B350 supposedly only have 2 dedicated sata and another 2 from repurposed sata-e lanes.
For more info on other stuff the chipsets adds like usb and sata see
https://www.gamersnexus.net/guides/2763-amd-chipset-comparison-x370-b350-a320
Hope that clears it.