"Heirloom" varieties that are sold are poorly maintained and extensively contaminated (often exceeding 50%). So not stable at all.

In order to properly maintain them you'd need closer to 1,000,000 plants with extensive but careful rogueing over over 30 generations with extensive genetic profiling to clean them up. They are all an absolute mess at this point.

Once you got them cleaned up the same 1,000 plants could maintain the population.