if you find one MB with 6 pcie + 1 or 2 m2 nvme slots + the use of pcie extender (1 to x4 pcie), depending on the config (motherboard+cpu= numbers of lanes) then you can achieve more than 6gpus, at least 7, 8 or even 9 or more who knows.
you will have to use m2 adapter to pcie and the pcie extender x1pcie to x4pcie
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so in this scenario your goal is to find gaming mb which has the more pcie available and m2 slots, like for instance this one:
- Asrock B450 Steel Legend AMD : x6pcie (2x16slot + 4 x1slots) + 2m2 nvme slots
- MSI B450 Gaming Plus Max : x6pcie (2 x16slot & 4 x1) + 1 m2 nvme slot
True, is really important the CPU lanes, right now you can get a Atlhon 3000g but only have 6 lanes, you can use it only with 6 gpu with the 4g decode option, the only way to max it is using a pcie extender x1pcie to x4pcie for a total of 9 gpu. (some ppl say that found a way to use 2 extenders on 1 motherboard, not sure about that, i need 2 adapters to test that). Remember 1 to 4 pcie adapters are pci splitter, they dont add more lanes... in other words it divide the bandwidth of 1 lane to 4.