They grow fast - they're hardy, and genetically inclined toward shrubby growth, so you'll need to prune to reduce the number of stems that sprout.

Let it grow without pruning for a year, then cut it back to about an inch from the soil line. It will back-bud profusely. Rub off the buds in places you don't want new stems & go from there.

They come easy from summer cuttings, too, so make more if you want.

If you like, I'll take a picture of a large-trunked tree I chopped back & am just starting on as a bonsai prospect. Here are a couple of other maples - just so you know you really can do what I suggested. ;-)

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Al

Thanks, I was thinking that was the way to go. I know I'll be hesitant to cut but I will next year. Love the bonsai.