Amaranth is a very high yielding plant. You can produce a shocking amount of it in an intensive setting, or even under just moderate conditions.
That being said, I hate to be frank, but you don't get something for nothing.
Often I read about amaranth and millet and quinoa as being great for growing dryland and in low fertility conditions. While that is true, they will produce only a fraction of their potential like this unless they're a land
You will likely need to mulch
, and/or weed in some way (mulching counts, at least for a while) in order to get a decent yield per plant.
Also, one reason these aren't popular in home gardens is the labor of harvest, especially for quinoa. I know a woman who probably still has buckets of it because she doesn't want to deal with processing it. I don't blame her. Still, it is a nutritious and good food.
I recommend Hells Canyon Millet from Adaptive Seeds. They're swamped with orders right now but I would consider buying some for next year.