Adapted from an older answer:
It's a myth that "only the richest American families" in southern states could own slaves, which is perpetuated mainly by Confederate apologists hellbent on arguing that the Civil War wasn't about slavery.
Spoiler alert: it was about slavery.
Many people in the antebellum South owned slaves. I wrote an answer on this awhile ago, adapted from the multiple times it's come up before. Before we get to that, here's a handy chart adapted from the 1860 census that looks at percentages of families who were enslavers, and percentages of enslaved people, in the seceding states and border states:
| State | Enslaver families by % of population | Enslaved people as % of population |
|---|---|---|
| Alabama | 35 | 45 |
| Arkansas | 20 | 26 |
| Florida | 34 | 44 |
| Georgia | 37 | 44 |
| Louisiana | 29 | 47 |
| Mississippi | 49 | 55 |
| North Carolina | 28 | 33 |
| South Carolina | 46 | 57 |
| Tennessee | 25 | 25 |
| Texas | 28 | 30 |
| Virginia | 26 | 31 |
| State | Enslaver families by % of population | Enslaved people as % of population |
|---|---|---|
| Delaware | 3 | 2 |
| Kentucky | 23 | 20 |
| Maryland | 12 | 13 |
| Missouri | 13 | 10 |
The answer I referenced above:
It's funny you mention Twain, because he classified lies into three categories: lies, damn lies, and statistics. That statistic you quote is used by Confederate apologists to make it sound like the war wasn't about slavery. (It was about slavery.) To answer your second question first, while many enslaved people worked on slave labor camps (Monticello, Mount Vernon, etc), many households in the South owned "only" one or two enslaved people.
Adapted from an older answer:
What percentage of whites owned slaves?
If you ask this question in this way, you'll get what seems to be a low number because it counts only property owners, who are heads of households, and not families/households who would benefit from the slave. (Think about it in this way: in my household I own a car, but my wife and child also benefit from my ownership of the car -- she drives it, he rides in it, we all use the groceries it brings home, etc.)
It also depends on whether you look at the percentage of slave owners (or slave-owning households) in the overall population of the U.S., or in slave states particularly.
So the better question is what percentage of households owned enslaved people. Here's a resource for the 1860 census which breaks down slave ownership by state: http://www.civil-war.net/pages/1860_census.html
I wrote about this awhile back in the context of a border state: http://www.reddit.com/r/AskHistorians/comments/2200dc/if_i_was_an_average_american_citizen_either/
To expand on that just a bit, the reason why how you count slave ownership matters is that if you state the question as "what percentage of whites owned people in 1860", you get a number that's about 8 percent of families owning enslaved people. (See the 1860 census link for context.) That number is often used by Confederate apologists to "prove" that the war couldn't be about slavery, because such a low percentage of households owned enslaved people.
So now we get into some basic stats. The 8 percent number is accurate, but it's also misleading, because fully half the states in the United States banned slavery, and those states held well over twice the population of the southern states -- about 22 million in the North, about 13 million in the South. Further, in the South, about 4 million of the population were slaves. So 22 million people were ineligible to own enslaved people right away, plus there were about four million enslaved people themselves -- so the question has to center on the population eligible to own enslaved people, that is, the remaining 8.2 million.
If you take a look through that census link it's quite illuminating; slaveholding families ranged from a low of three percent of the population in Delaware, to highs 46 percent in South Carolina and 49 percent in Mississippi. It's also interesting to look at enslaved people as percentage of population, where 57 percent of South Carolina's population and 55 percent of Mississippi's were enslaved people.
For more on this, see also these threads:
https://www.reddit.com/r/AskHistorians/comments/2200dc/if_i_was_an_average_american_citizen_either/