A family of four will probably want to grow between 10 and 20 plants per person, for a total of 80 plants. Certain crops require a bit more room while others are perfectly fine being planted in close relation to one another. The size of the family and how much they would need to eat to survive comfortably is a determining factor as well. Larger families comprised of mostly adults will need more than smaller families with young children.
Greenhouses are built to fit the needs of the people they are providing for. It also matters what you'll plan on growing that would determine how big the greenhouse has to be built to. For example, carrots are a great source of vitamins and are able to be used in many dishes but require a considerable amount of space to grow.
