re: Society breaks down and you have to make it in a subsistence agriculture situation

Not sure an exact figure, but it wouldn't be a very big area I don't think. I think you could feed a family of 4 on maybe 3 or 4 acres maybe. That should be big enough to raise some chickens and a few pigs and have a decent enough garden for fruits and veggies. Idk maybe 5 acres and still have room to rotate crops every other year where you don't deplete the soil.

ETA you could definitely do it on less than 5 acres but 5 acres would be big enough to give you some options and variety where you aren't eating the same 3 or 4 things every day. Something that size would let you keep living without making a huge sacrifice in you quality of life I think.