Make yourself a list with three columns. In the first column, list the heirloom vetables that you absolutely have to have. In the second, list the ones you really would like to grow and in the third, list those that “would be nice.” Then consider how many people you will be feeding. Do you want to do some canning and freezing for the winter? If so, you will want to plant more.
Here is an idea of how much you should plant for 4 people.
Beets — 10-15 feet of row
Broccoli, Brussel sprouts and cabbage — 10-15 plants each
Carrots — 20-30 feet of row
Corn — 20-30 feet of row
Lettuce — 10-15 feet
Peas — 30-40 feet
Pumpkins and Squash — 1-3 plants each
Radishes — 4 feet
Tomatoes — 10-15 plants
Here is good planting information http://downtoearthseeds.com/plantinginfo.php.
The production of your organic garden can vary alot. There are many factors that influence production. How healthy is your soil? How much rain do you get? Do you irrigate? Were the weeds allowed to grow until they competed with the heirloom vegetables? Did you get the garden planted on time? Do you harvest in a timely manner? Experience will help in all these areas. You don’t have to be a rocket scientist. Talk to other gardeners. Go to your local farmers’ market and talk to growers there. Ask them about your problems and how they deal with them. Visit their farms. (But they are a hard working group, so don’t take up too much of their time.) Or talk to the neighbor that gardens in your area.
Books can provide alot of answers. Get at least three or four books on vegetable gardening. Heirloom vegetables are grown just like hybrids. Planning can make a world of difference. We use leaf mulch to cover all the gardens. Consequently, I do very little weeding. The soil stays cooler and more moist. Each spring, we install drip tape for irrigation. The tape lasts for several years if taken care of, and it makes irrigation as easy as opening a valve. That is all on the flat garden. I still use overhead sprinklers on the raised beds.
Remember, in gardening, there are no failures; only learning experiences and success.