Answer: In the garden, spacing is often based on the expected mature width of the plant. For instance, a marigold variety growing ten inches wide would be planted about ten inches apart measuring from stem to stem or "on center". For a thicker look, they might be placed about eight inches apart, for a look with soil showing between the plants the gardener might put them about a foot apart.
For containers, however, there is no rule of thumb, except to say the plants are usually crowded a bit to give a very full look sooner. The plants are also coddled with ample water and fertilizer to keep them at their prime because the containers tend to be a focal point. (You do not want them so crowded that they cannot get enough light, water or nutrients, however.) Since it is a container you can also remove, add and rearrange a bit during the season to keep things looking their best.
I'm sorry I can't be more specific since the guesstimate would be based on the plants you want to use.
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