It depends on a whole bunch of things, not the least of all what specifically you are talking about. Mainly, the idea is that if you plant too many plants together they will not have room to grow to maturity. You can fit 3000 broccoli seeds in one square foot, but you can't grow a yield of 3000 broccoli plants per square foot.
Think of the specific plant and how much space it takes. This is Zone 1
gardening, not to be confused with a food forest or anything less labour intensive. And if you are planting seeds in place, they generally need bare earth to start off well, just like the weeds do. They are all plants.
I tend to plant thickly and eat the thinnings or transplant them around to grow on. I call it "square inch gardening"!