For most of the crops we grow in our ZipGrow Towers - largely nutrient dense greens and herbs - we stick to about a 6-8 inch spacing. This inch spacing helps us grow 8-10 basil plants in a single 5' ZipGrow Tower.
The number of plants that can fit in each ZipGrow Tower can be derived from their height.
3ft - 4-6 lettuce-sized plants
5ft - 8-10 lettuce-sized plants
7ft - 11-14 lettuce-sized plants
Other types of plants will require more specific measurements:
Mint, for example, grows like a weed and spreads throughout the entire Tower. This differs from the majority of your greens like lettuce and kale that tend to grow more centralized and gain the majority of their weight in one spot.