Well, in answer to this and the other question someone posted (FC?) about what would be necessary to support a family of four, it TOTALLY depends on your area, soils, climate, and what kind of life you wanted.
There are plenty of people nowdays going "off-grid" and living "sustainably" and such. The Amish basically live this way, and usually sell products as well for extra money. There are SO many variables though that it's pretty much impossible to give a "pat" answer...
In our area near Houston, we're running our cow-calf operation at about 2 acres per head. Of courser we're also baling our own hay, and I lost a field of about 15 acres that I was cutting hay on once or twice a year... so now we're having to take hay off our own farm by setting aside one 20 acre field. We're actually a little lower animal density than that, because currently we have about 35 head on 87 acres at Needville, but that varies, because we sold about 15 calves a few months back and have had a few calves since, and will continue to throughout the winter and spring (we don't do a set "calving season"-- we let the bull run with the cows all the time and breed them as they cycle-- simpler than keeping the bull out and making the cows all "cycle together" so they all breed together and calve together, like they do up north (to avoid calving in winter when the cold weather can kill the calves). One of the benefits of living down south!) Of course, in the drought, we had about 40 head on 87 acres and the whole place looked like a putting green they were grazing it so hard, so it really is TOTALLY dependent upon the weather!
In Shiner, just 100 miles west, we're running about 50-55 head on 160 acres... so over 3 acres per head. That's about as high as you can realistically go with any sort of sustainability, and basically 4 acres per head would be better. In a drought, no way you can sustain even that.
Brood cows have lower nutrient requirements by far than dairy cattle... so if someone's running a couple dairy cows on a couple acres, well, they must be living in paradise-- just perfect grass and excellent climate, rainfall, etc...
It CAN be done, but it's SO dependent on local climate, soils, methods, etc. as to be meaningless to discuss in a broader sense-- what works on a place like that might not work AT ALL just a few miles down the road on different soil, or in a year when the weather doesn't cooperate...
I know it's been a real education for me helping my BIL in Indiana with his soybean and corn crops, and fiddling with hogs... (we had a hog or two for our own meat when I was a kid, but Dad quit bothering with it by the time I started school in '75). The way they do things and the way we do things are VERY, VERY different... it was as much of a curiosity for them when my wife and I married and they first came to visit us, when I was row-cropping cotton and grain sorghum, and seeing the soybean and corn farming in our area (which is very much secondary crops). The methods are DRASTICALLY different!
Later! OL JR