As I slowly work my way beyond the seed-saving basics, I'm starting to realize that saving the seeds from the lone kale plant that overwintered in my garden might not be a good idea.  To understand how many individuals you need to save healthy seeds, first you have to delve into a bit of pollination biology.

All plants fit somewhere on the spectrum between strong inbreeders and strong outbreeders.  At the far inbreeding extreme lie plants like tomato-leaved tomatoes, peas, and green beans, all of which have flowers that make it extremely unlikely that an insect can get inside to move pollen from one plant to another.  These vegetables breed with themselves as a matter of course, so they've worked the bad recessive traits out of their populations.  The pros recommend saving seeds from at least 20 plants for strong inbreeder species, but at the backyard scale, you can get away with just a few parent plants (or sometimes even one.)

At the opposite end of the spectrum, strong outbreeders (like most crucifers, swiss chard, spinach, and beets) depend on getting pollen from another plant to keep those recessive genes in check.  You'll need around 80 to 100 individuals of these species in your garden to keep your seed vigorous.  If you save seeds from just a few individuals, next year's plants will tend to be weaker and you'll eventually need to buy seeds to reset your gene pool.

Most plants, of course, lie somewhere in the middle, and there are also outbreeders (like most cucurbits) that don't seem to show many problems even if you inbreed them.  You can download a free pdf here that gives recommended population sizes for most common vegetables.  (The website asks for your email address before download, but they haven't sent me any spam.  The file you want is A Seed Saving Guide for Gardeners and Farmers.)  Alternatively, if you want to become a real seed-saving pro, I can't recommend Seed to Seed by Suzanne Ashworth highly enough.  This text made the very short list of book purchases in our household for 2012 and has turned my dabbling in seed-saving into a scientific endeavor.