AM5 CPU's have 28x PCIE 5.0 lanes and it doesnt really matter what configuration some mainboards allow with PCIE lanes, bifurcation and bandwith reduction will be used if you hit the 28x PCIE 5.0 bandwith with components.

• 16x go to the MAIN GPU slot
• 8x go to the first 2 CPU_M.2
• 4x are used by 1 or 2 chipsets (x670E / x870 / x870E have 2 PCH chips).

The 16x GPU lanes are usually only impacted by 8x8x bifurcation, if you use more than just two M.2's.

A few x670E boards allow you to use four M.2 storages AND still keep the 16x GPU lanes, but most X670(E) boards and the even worse nearly all x870(E) boards with USB4.0 REQUIREMENTS will cut after 3 or 4 M.2 into the 16x GPU lanes.

USB4.0 is most of the time the reason why you even need to check mainboard manuals for bifurcation and some mainboard layouts with PCIE configurations are quite honestly just stupid for a gaming system.

While we could argue that two M.2's are enough for everyone, M.2 capacity did not change that much over the last 5 years, while games and data growth skyrocketed in storage requirements - it is not that hard to need 3-4 M.2's especially if you want the value and not the 4TB variants.

Overall AM5 is fantastic compared to Intel when it comes to PCIE bandwith, but a lot - A LOT of AM5 mainboards are not designed for 4x storage without touching the GPU lanes. If you know this, you can just filter out all the stupid AM5 mainboards and if you are fine with just 2x M.2 you dont have to care at all what board you get for reasons of PCIE lane configuration.