you will hardly find a gaming motherboard ready to accept 9gpu BUT
if you find one with 6 pcie + 1 or 2 m2 nvme slots + the use of pcie extender (1 to x4 pcie), depending on the config 'motherboard+cpu) then you can achieve +6gpu, at least 7, 8 or even 9 or more.
you will have to use m2 adapter to pcie and the pcie extender x1pcie to x4pcie
.
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