How Much Harvest?

If you are new to gardening and want to grow some food this is the post for you.
Planting a tomato plant or two is nice and you get a few tomatoes, if you don't kill them somehow, but how do you know how much is right for you and your family?  When space is at a premium this can be an important decision, it can determine whether you will have room for tomatoes, carrots, zucchini, bell peppers and cucumbers... or just enough room for tomatoes.  Having too few tomatoes may mean you have enough for hamburgers one night, but not for salads the next day.  Or that you harvest more than your family can eat fresh, but not enough to make it worth the effort to can salsa.  Having too much of one harvest can mean an overstock of salsa, spaghetti, canned tomatoes, sun-dried tomatoes, tomato soup, fresh eating tomatoes... you get the idea.  You don't want to waste garden space growing more than you could possibly eat in a year.  Most home canned goods and frozens are only good for a year, and by the next summer you'll have a whole new crop to process.
Tomatoes are a good example for this.  I have a family of four that is not big on vegetable eating, my son is a huge cherry tomato eater, my husband and daughter prefer their tomatoes fresh in their meals, while I like to have salsas, sauces, canned tomatoes, soups, etc. for after the harvest.  My 2011 garden worked out just about perfect giving me a mix of cherry, saucing and slicing tomatoes at a rate of about 4lbs every 4 or 5 days.  When we had an overabundance of slicers I simply seeded them and added them to sauces, when saucers were more productive I would chop them into meals.  The cherry tomatoes were eaten fresh or added to salads.  In total we planted 12 tomato plants with a mix of extra-early to late season, in differing sizes shapes and colors.  We actually ended up short on spaghetti sauce but I gave a lot of tomatoes to my mom because I was sick of processing them.
The tomatoes worked out to 3 plants  per person, which seems to be just right for my family, though not necessarily for yours.  If you are big vegetable eaters, or plan to share a lot of your harvest aim for closer to 5 per person.  If you are overwhelmed with that number you can always post on craigslist that you have too many tomatoes and would love to make a trade with someone who grew too many bell peppers, or share your extras with friends and family.  Around here, summer squash are the most overgrown vegetable (people just don't realize how productive they are) and are nearly impossible to get rid of, but tomatoes, carrots, peppers, corn, and peas are an easy thing to trade with friends and neighbors.
Generally when I'm selecting a new food to grow I put in one or two (if needed for pollination) plants and wait to see what comes of it.  This served me well when my brother insisted I should grow five or six eggplants last year and I decided to leave more cucumber space instead and only plant 2 eggplants.  Those 2 eggplants produced more eggplant than I could have ever managed to eat, luckily my mom took a lot of it off my hands, and meanwhile the extra cucumber space was needed since my pickling cucumbers ended up being completely shaded out by the nearby tomatoes.  For new gardeners this is a great way to practice trial and error too, and may keep you from giving up in disgust when your favorite veggie does poorly the first year you plant it.  When planning your garden, plan many different types of vegetables, in several varieties if possible, plan for them to ripen at different times if you plan to eat them fresh, or all at once for processing big batches.  As exciting as a lush patch of hot chilis may sound, if you won't eat them or don't know someone who will, it's probably not worthwhile to grow them.  And obviously, if you are a massive watermelon eater who simply can't get enough it may be worthwhile to plant several just for yourself.  Keep in mind that perennial fruits and vegetables tend to get bigger or spread over the years too, don't keep investing in blueberry bushes or strawberry plants to fill your garden bed if a few will spread in a year or two and fill it nicely.  Instead use that money to purchase some u-pick berries the first year or two until your patch can keep up with your demands. 
As an example of what can be done in a small garden this is a list of what I planted last year in my 102 square feet.
-one Thai chili plant
-two jalapenos
-two lemon cucumbers
-four slicing cucumbers
-two eggplants
-one small melon
-one luffa
-three pumpkins
-one zucchini
-six marigolds
-four calendula
-about 30 runner beans
-12 tomatoes
-eight pickling cucumbers
-three bell peppers
-eight basil
-six garlic
-24 bush beans
-six mammoth sunflowers
-six celery
-three cabbage
-four broccoli
-24 lettuce
-27 spinach
-12 nasturtiums
-42 peas
-and a handful of carrots

That's over 200 individual plants, of 27 different types of vegetable/flower.  Admittedly there were only a few of each type but they were more than enough to keep my family stocked for the year, with a few exceptions.