I know Real Seeds recommend something crazy like 20 plants if saving brassica seeds, which is why I have no plans to do so! They reckon that you get too much inbreeding unless you have a decent number of plants.
https://realseeds.co.uk/brassicaseedprocessing.html
https://realseeds.co.uk/brassicaseedprocessing.html
How much veg and wildlife can I pack into a 6m x 8m garden in suburban Cambridge? Let’s find out! 
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