8 is as many as I could fit on that freehub with the 9-speed spacers. I’m guessing the 10-speed must be narrower again? I’m quite happy with this because it means I can squeeze 8 gears instead of my current 6 into a 126mm rear with no re-dishing and (what looks like) no clearance issues.
That works up to 10 speed shimano/sram on Shimano 7 speed hubs, you'll need the same speed shifter for it to work.