If you plant Japanese Maple seeds and they sprout, you will have Japanese Maple seedlings, no more, no less. If you are refering to named cultivars of Japanese Maple, the plant must be propagated by an asexual method (cloned if you will) such as rooting a cutting, or grafting. Seeds taken from named varieties may or may not resemble the cultivar from which they are picked, but at most are "seedling from Garnet" rather than a replication of Garnet, even if they're a dead ringer for the parent plant.