I would say 1 cubic yard of manure per 20 square feet (or one cubic yard per 100 square feet). This assumes the bed is 10 feet wide and the rows are 3 feet apart. You could also calculate it as 1 cubic yard per 25 square feet.
If you have an average of 10 square feet per row, that's 10 times as much manure per row than per square foot. If you want to grow a lot of veggies, you'll need more than 1 cubic yard of manure per 20 square feet or 25 square feet. I would guess that you'd need at least 3 cubic yards per 20 square feet or 4 cubic yards per 25 square feet, just to have enough nutrients in the soil for the veggies to grow well.