The number of plants needed varies. It's not the same for all vegetables. I don't have the information available right now but my reasoning is the same as yours regarding beans. I don't worry about how many plants are represented in my saved seed. But then, I don't consider myself a careful guardian of any strain either. I figure, if one of my beans deteriorates, I can get new seed to replace it. None of my varieties are so rare they can't be replaced.

I doubt you will see a loss of viability from saving beans over several generations. What is more likely is that there would be some changes due to mutation or cross pollination. Those doesn't happen often, but over a long enough period of time there is some probability of it occurring.

As Carol Deppe says in her book, Breed Your Own Vegetable Varieties, all seed saving is plant breeding because there is always some selection involved, whether intentional or not. In that sense you are right to be concerned about maintaining a true strain. But, if I understand it correctly, beans are one of the easiest to maintain.

Jim

Bill Best at the link below tells about the NT Half-Runner coming from a single plant. In other words, a single seed.

Today a lot of people grow it and it is a high quality bean. - Dick

Here is a link that might be useful: Bill Best

Thanks for the input. It seems I was mostly right in my thinking. I usually grow 24-30 plants anyway, but this question was nagging at me.

This is probably more important when working with a sample of a landrace. I have a "variety" I call Mecatlán Black, a non-vining half runner from a village of that name in Veracruz, Mexico. The original sample of seed had quite a variation in the shape of pods, etc. So, when I plant it for seed I try to grow as many as I can, in order to maintain the variability.

George
Tahlequah, OK