https://en.wikipedia.org/wiki/Historical_racial_and_ethnic_demographics_of_the_United_States
Apparently we got up to 21% back in 1770. Free blacks represented 8 to 13% of the black population (Free black chart is a little over half way down the page). So about 2% of total population and that drops slave labor below 20%.
Then as immigration and population grew we dropped down until it hit 9.7% total black population in 1930, before it started slowly growing again reaching 12.6% in 2010.
Blacks certainly contributed to American growth with some hard back breaking work. But I don't think 10% to 20% of the population did most of the work and supported the other 80 to 90%.
“But I don’t think 10% to 20% of the population did most of the work and supported the other 80 to 90%.”
Eugene Genovese’s “Roll, Jordan, Roll” and “Time On The Cross” by Fogel & Engerman are said to be good studies of slave life in America. From what I recall reading, a lot, maybe most, slave holdings were small operations with the masters having to work as well. Plantations of course were an entirely different matter. Plantations like Monticello were small cities.
Slaves were expensive. In New Orleans historians point out that canals there were dug by Irish immigrants. Slaves were certainly capable of doing the work, but it was dangerous work and slave owners weren’t about to risk ‘their people’.
Another book that’s worth reading is “Weevils In The Wheat”, interviews of elderly former Virginia slaves conducted by black writers during the Depression.