One way to decide how much to plant is to estimate how big your garden should be, then fill it with food you think you'll eat, as suggested in the Square Foot Gardening method. According to this method, they suggest that about 50 square feet for each adult could provide enough vegetables during the growing season and some for preserving. John Jeavons, in How To Grow More Vegetables, recommends about 100 square feet per person to grow most of their vegetables for the year. Another 100 square feet would, according to him, allow enough room to grow all that person's food for the year, including grain. These numbers only include actual gardening space, not aisles, and assume that you're using an intensive, or wide-row, method.
But how many plants of broccoli will produce enough for your family? Everyone seems to have different ideas, but these numbers will give you a place to start. Try planting less the first year, unless it is something you know your family loves to eat. Most of these numbers would probably be plenty for fresh eating and preserving for the rest of the year.
Asparagus: 10 plants
Beans and Peas (all types): 8-11 square feet
Broccoli, Brussels sprouts, Cabbage, Cauliflower, Celery, Chard, Endive, Kale, Peppers, Tomatoes: 5-7 plants
Cucumbers, Head lettuce, Rutabagas: 10 plants
Eggplants, Melons, Okra, Pumpkins, Summer and Winter Squash, Watermelons: 3-4 plants
Onions, Garlic, Leeks: 4-7 square feet
Corn: 50 plants
Spinach, Leaf lettuce, Greens: 3-4 square feet
Turnips, Rutabagas, Radishes, Kohlrabi, Carrots, Beets: 3-4 square feet
Sunchoke (Jerusalem artichoke): 5-10 roots
Strawberries: 15-20 plants
Herbs: 1-2 square feet