the board itself is electrically wired so that as soon as you use a M.2 in the Gen 5 M.2 slot, you'll be limited to x8 for the GPU. In total, you can install up to 6 M.2s without any issues, just keep in mind that only 2 will be directly connected to the CPU except you manage to get your hands on an x8 card which supports 2 M.2s. That way, you can in theory get up to 7 M.2s into the system.
Even though all slots from the PCH are Gen 4, Raiding multiple devices on those slots won't give you much performance benefits as they only have a single connection to the CPU itself.
DIMM M.2: 2 M.2s PCH PCI x4: 1 M.2 PCH and one of the mainboard M.2 slots is also connected to the PCH. That's a total of 4.
CPU itself, you could use the first PCI Gen 5 slot with an add-on card for 2 additional M.2s. In this case, do NOT use the onboard Gen 5 M.2 slot. Furthermore, there is one more Gen 4 M.2 slot on the board connected to the CPU.
That should give you a total of 7 M.2s. I sadly only tested a total of 6 M.2s myself so far as I got into a few troubles with the custom loop design to actually move the GPU to the second slot in the end.
From what I have researched, running the GPU at x8 is not much of a performance hit.
I have the z690 rog maximus extreme with a 3090ti, and all 5 m.2 slots populated. I have confirmed with technical support that the m.2_1 slot, when populated, causes the pcie_1 slot to drop to x8 link width. And it also disables the pcie_2 x16 slot.
I have decided to leave the GPU at x8 with the m.2_1 slot populated. Hope this helps. B.