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.