vegan

what do you think would be the minimal amount of animals and land required to feed those 8 billion organically

It's a question complicated by the current "realities" of capitalism where, for example, people are willing to have their food grown in terrible conditions in another country and then imported over thousands of miles and many weeks to save a few percent on costs.

If we can ignore all that and I can just spell out what I think is the ideal scenario, it looks basically like this:

• much less meat consumption overall, closer to 50 kg/person/year vs the current 124kg (wtf?) for Americans
• instead of consolidation into huge monoculture operations, many much smaller but highly diversified farms - ideally you should never be more than a few miles from a farm. I include in this urban areas where we set aside land and rooftops for this purpose.
• Land for farming should be free and granted to farmers for their long-term use.
• All farms should be highly integrated so that any animal production there is happens in conjunction with vegetable production. You need more land for this so you can do things like rotate a field from sheep for a year to vegetables the next
• All farms should be using agroecological practices with no artificial fertilizers, minimal tillage, etc.
• Processed foods that rely on huge volumes of commodities like corn would not exist. Corn for ethanol? Probably not either.
• Farms should leverage collective labor where possible and we should bring agricultural education into the mainstream AND work to remove the biases against farming that are so evident here.
• Community composting efforts, including humanure, would help reduce the dependency on animals for fertility.
• One of the things that makes this all viable is the higher nutritional density possible from shorter supply chains.

These are a lot of preconditions, I acknowledge.

If this can happen I believe we will require less land than we currently use for our combined animal & human food production. Currently that's around 50% of the land area of the US, or 1.2 billion acres but this includes about 650 million acres for grazing and about the same for forest-use, which I believe means lumber production but could also mean grazing. Only about 400 million acres are used for growing crops and the majority of this is for commodities like soybeans and corn.

The best proxy for productivity is not actually yield of commodities but dollar value. This is because we want to equate as closely as possible productivity with nutrional value without the value-adds of say, processed foods. A typical acre for commodities might net you $150-300 per acre in profit while a typical acre on an integrated farm growing diverse produce (aka speciality crops) can yield over $25,000 per acre up to as much as double that. If you remove some of the headwinds from farmers like mortgages, competition with cheap imported produce, high marketing costs and high transportation costs you could probably see much more than that.

In this scenario food costs would very likely stay about the same or even go down because farmers can get very good margins and better economies of scale from higher demand.

So, back to the question of how much land - if you use the dollar amount and take $2 trillion as the figure we spend on food, and assuming margins around 30% that means revenue of $100k per acre. This would mean you'd need about 20,000,000 acres. Even assuming my napkin math is way off and it's double or triple that... it's a lot less.