This depends on how large a population you start with and how many plant are grown on a yearly basis. To simplify a lot, grow about 200 plants every time it is grown for seed and inbreeding depression will be minimal. As an example, lets say you want to save the best 200 plants in a field where 1 in 5 plants is good enough to keep. You will have to grow 1000 plants to retain 200 with desired traits.
You can grow a large population of corn in a smaller space by growing some each year and freezing seed each year. Every few years mix the seed saved from different years. For example, sow 1/5 of your seed in year one.
Take Darjones numbers from above as an example. Save seed from 40 of the best ears. Not all the seed from each ear, but rather 1/5 as much as you will need to replant in the next cycle. Each of the next 4 years do the same. After 5 years, mix all the seed you have saved. That will be the same as if you had grown 200 plants in one year.
For what its worth, I have had good results with much smaller number than Darjone and many other recommend. I am comfortable saving 50 ears per generation. Most people think this is too few, but I'm not having a problem with inbreeding.
Corn breeders who have worked with corns saved for centuries in small isolated fields at high altitudes, found that those landraces show little or no inbreeding depression, nor hybrid vigor when outcrossed. In short, they have adapted over centuries to gradual inbreeding and are doing fine. That is not to say that there weren't problems in the first century or two.
I use to work on the river that deliverance was filmed, and have run multiple times the other river (Tallulah) it was filmed on. the companies first boats actually came from the boats they used to film it. The banjo player worked at the local Walmart there last I heard (it was a while ago). One of the few times I swam, as an extreme kayaker, was at a rapid called "deliverance rock", where they show the last time his hand slipped under a rock, it was at flood stage, and their was a massive hydraulic. Another notable place on that river is "sodomy beach" where the sodomy scene was filmed, it's a campsite just above the most difficult section, of section IV of the chattooga.