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

350 acres. Duh

One. I'm going to eat a lot of seafood and seaweed.

beef, poultry, swine, or gardening?

700 sf for each person

My papaw(and mamaw) had about an acre garden that fed my uncles family(4), my dads family(4 of us) and my aunts family(3) for the year. corn, beans, peas, okra, squash, tomatoes. Everything we needed sides wise.

With hunting and fishing I think two acres would do it.

Everybody saying 1 acre or less is forgetting about the necessity of raising some kind of animals for protein, and the necessity of having a cash crop to sell/trade for things you can't grow or make yourself. Paw Paw and MeeMaw may have had a little garden on the side, but they also had some other source of income. You could get by for a year or two with a big garden. For long term survival you're looking at 40+ acres, and the skill/equipment to scale up a regular farming operation. Almost no one who doesn't farm for a living would know how to do that, and even most farmers would be lost trying to plow with a mule, if they could even find a mule drawn plow and a mule to pull it.

You can grow as much food as you want. It's protecting it that's the real challenge.

I think I could do it on 1 acre with only chickens as livestock. It'd take some planning and some vertical gardening for some stuff. You could plant corn, peas and squash together. Rotate to turnips and radish in the winter. It would be difficult on only 1 acre, but I think I could do it.

But if society breaks down, guarding all of it will be the biggest challenge.