A rule of thumb for planting density would be plant each stem or crown about its leaf length apart. The plants will crowd themselves but initially the lower part of the plants will have plenty of room. So a sword plant with 6" leaves would be planted about 6" from another sword plant. Since the sword plants combined would have 12" of leaves they won't look too far apart or too crowded. A stem plant with 1" leaves would be planted 1" from another stem plant. Then adjust to your taste, Perhaps 6" is too close for you. Perhaps you like that particular stem plant 1.5" from its neighbor. I am sure some picky plants MUST have certain conditions but this should hold for the easier plants.