One of the hardest heirloom crops to keep pure and uninfected by GMO's is corn or maize. The first and probably the most important rule is your stand size. Maize plantings cannot be less than 40 plants. This is the absolute minimum and double would be better. There is no way that one can effectively save seed from year to year of an OP maize variety with less than 40 plants.
If you do, you will find that you will have destroyed the genetic variation essential in heirloom and OP corn varieties that can never be recovered.
Also with a stand size of less than 40 plants you will also find that a high percentage of your harvest will be poorly pollinated and kernels will be missing on the cob.
Fortunately, we at Livingseeds understand that besides just getting a crop to eat, many of you would like to save your precious corn seed from year to year. We supply a minimum of 80 seeds and often well over 100 seeds, so that you have the means in every single packet to enable to save your corn seed from year to year. Supplying you with too few seeds from the outset will put you at a disadvantage in terms of seed saving.
Please have a look at this link, Keeping Heirloom Corn seed pure for more information on how to plant and save your seed.