If you want to create a dense bushy crop, just trim regularly at different heights as the plants grow... 2 or more stems will grow from each cut, here is a diagram:
Diagram from Google Images.
If you find that the bottoms are getting very old and most of those bottom leaves have melted, just cut the heathy tops, uproot and discard the old bottoms, then replant the healthy tops. You'll usually need to do this on a regular basis to keep the plants looking fresh.
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