It depends on how many people you intend to feed, and how large your greenhouse is. How big is your family, and how much will you be supplementing their diet? If you are merely looking to add a few sides at dinner time, you can plan to allocate about 5 or 10 plants per person, per year. If you are looking to fully feed your household, expect to grow between 20 and 30 plants per person each year.
You'll need more or less plants for each person depending on the plant you choose, and what their individual yields look like. For example, each person will only need about one pumpkin plant per year. Potatoes however, you may need closer to 30 or 40 of to fully replace all the ways potatoes are used in their diet.
