I always plant too much. It's some kind of sickness. I plant like I'm feeding my entire town of 170 people. Of course, my soil still needs improvement, so my yields are not great - and that is part of why I plant so much. Still, I often wonder why I think 40 tomato plants is a good number when we don't eat a lot of tomato sauce, or what I'm going to do with 150 pie pumpkins. I answer myself in fall when I spend many hours canning, freezing and otherwise preserving the harvest.
Still, it's nice to figure out how much you really need - not just for eating during the season, but how much you might want to grow if you plan to preserve food for the winter and eat from the garden for the entire year. Start here, planning how much your family will eat of which foods, and plant a garden that's not too much, not too little, but just right.