Tree and Land penjing the tree is the main
artistic expression.  Any rocks, accent plants
or other scenery added is only to accentuate
the tree.  1 to 3  trees is more common.  5 or
7 trees is a sparse forest.  And 9 or more is a
dense forest setting.

Rock penjing the rocks are the main artistic
focus.  Added plants are only to accentuate
the rocks.

Water and Land penjing the trees and rocks
are more or less of equal importance in the
scene.

Penjing can be as large as 5 feet long with
trees or rocks as high as 2 or 3 feet.  My
personal preference is shohin size with
containers about 18 inches or less in length.
There is usually a dominant and a secondary tree in Tree and
Land and Water and Land sceneries.  Similarily a major and
minor rock in Rock landscapes.