That depends on the type of plant you are growing, but in general you can plant about twice as densely in aquaponics as you can in a traditional soil-based garden. This is because your aquaponics plants are getting exactly what they need at the root zone (nutrients, water, oxygen) so the plant doesn’t need to send its roots out searching for these things.  The result is extremely compact, healthy roots and less competition for nutrients and water below “ground”.  Your limiting factor is really how much light is getting to the plant.  Check the space recommendations on the seed packet to get a better idea of how much room your plant will need at the full grow-out size, and know that these spacings were typically made for soil gardens so you can likely plant them closer.  Just keep in mind that airflow/circulation is an important aspect of plant health AND pest control, so give everyone enough room to breathe freely!

To calculate heads of lettuce per week in deep water culture, divide the total number of plant spaces by the culture time (5 weeks is average) to get the number of heads you can theoretically harvest each week. This means that every week you need to replace the harvested heads with a new group of seedling transplants. In managing this rotation you would have 5 different weeks of growth in the system with a new group ready to be harvested each week. The key is to continue planting the same amount of seedlings each week so you are consistently replacing the harvested crop. Again, results will vary and these estimates are just for planning purposes.

Beware of companies selling aquaponics systems that set unrealistic expectations of how much food their systems can grow! Even though aquaponics does generally experience fast growth rates, it is, in the end, still a natural system bounded by natures rules.