I have purchased 12 sapling "sango kaku" japanese maples - I believe they
are also called coral bark. I intend to do a forest planting on slate.
BRBR

I wouldn't plant Japanese maples on slate. They will need very frequent
watering. I would plant them in a group planting tray.
You want an odd number of trees. Keep the extra one in case one dies.
Some of the books have diagrams of various number group plantings. Peter Chan,
Bonsai Master Class does.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)