The answer to this question somewhat depends on what you mean by 'owned'. In the strict legal sense of the word, the 1860 census reported just shy of 395,000 slave owners living in the antebellum Southern United States, which represents approximately 3.2% of the population of the South in that period, or just 1.2% of the population of the total United States. Superficially, that seems like a very small figure that makes slave-holding a rare and marginal practice in white society - this is a contention often made by Confederate apologists and white supremacists. The reality is that figure dramatically understates the significance of slave holding in the United States in 1860.

First and foremost, the count of 395,000 is a count of individual slave owners - people, usually men, who are recorded as owning in law one or more slaves. That count does not consider how many individuals had direct access to, and benefited from, slave labour. On average throughout the South, 27% of free family units had at least one slave owner in them. Across the Lower South where the plantation economy was at its strongest, slave owners belonged to well over one-third of the proportion of families; in South Carolina and Mississippi, very nearly half of all free families owned at least one enslaved African American person, meaning that more than half of the white population directly benefited from slave labour. Although the vast majority of these slave owners were not plantation owners - most were small farmers - most slaves did live and work on plantations. If the head of a household owns a slave, it stands to reason their spouse, children, close friends and other close relatives all stand to directly benefit from slave labour.

This is to say nothing of people who benefited from slave labour despite not being in a slave owning family themselves. It was a widespread practice in the southern United States for slave owners to hire out the services of their forced labourers for a wage or fixed price (payable to the master not the slave), and in cities and towns enslaved people were employed in all manner of trades and public services that non-slaves benefited from. Although from the perspective of African American people plantation life and work were the normal condition of slavery, white slave owners and beneficiaries employed slave labour in every aspect of Southern life imaginable - in agriculture, in industry, in commercial enterprises, in artisan trades, in rubbish collection, in domestic service and so on. We must likewise consider those who were intimately involved in slave exploitation in the plantation or urban economy without necessarily being part of owning families: overseers, satellite farmers, slave traders and patrollers, doctors and preachers, etc etc.

Something else that must be emphasised is the extent to which slave ownership was a financially and economically privileged institution. The cost of purchasing even a single enslaved person was enormous; in the 1850s, you're looking at well over $1,000 to acquire a single male fieldhand. In terms of its purchasing power, $1,000 in 1855 is about $21,000 today; it's true value, accounting for differences in quantity and distribution of wealth, is closer to $390,000 (in other words having $1,000 in 1855 will buy you as much as $21,000 today, but having $1,000 in 1855 makes you about as wealthy relatively speaking as having $390,000 today does). Consider this account of a slave auction in Alabama in early 1854:

Woman and small child, $1050; man aged 19, $950; man and wife aged 18 and 17, $2000; boy aged 14, $640; girl aged 10, $525: man aged 24, $860; boy aged 11, white, $585; boy aged 11, white, $625; woman aged 25, $900; man and little boy, aged 50, $1020; woman age 46, $395; man, with the gravel , aged 19, $700; man, perfect aged 40, $1600; woman 40, girl 8, $600; man aged 27, $1410; boy aged 12, $725; girl aged 4, $300; girl good looking, aged 14, $855; girl, a little blacker, aged 15, $845. [from The Liberator, "From the Car Leader", April 21, 1854]

^N.B.: ^this ^is ^from ^a ^radical ^abolitionist ^newspaper; ^two ^boys ^are ^referred ^to ^as ^'white' ^because ^they ^are ^lighter ^in ^skin, ^but ^they ^are ^still ^African ^American. ^It's ^a ^rhetorical ^tactic, ^drawing ^attention ^to ^the ^hypocrisy ^of ^Southern ^white ^racial ^ideology ^and ^to ^sexual ^abuse ^in ^slavery.

Even a little girl of just four years old cost $300 - almost $9,000 in terms of modern purchasing power, and much more in terms of relative wealth. The best possible workers, from whom an exploiter could gain immediate economic benefit, cost well in excess of $1,000, with prices consistently rising right up to the Civil War. This was not an institution that it was easy to just walk into even as a member of the white middle class; although it could be and usually was enormously profitable to invest in slavery, the upfront costs were not insignificant if you did not inherit slave labour. It is not remarkable that so few individual white Southerners owned slaves - it is perhaps remarkable that so many families did given the financial barriers. Slavery was an institution that brought with it not just the potential for enormous profit and the mitigation of one's own work load but also the prestige and social privilege of visible wealth.

For many non-slave holding white people, especially white men, one day being a slave owner was something to aspire to - owning slaves was a symbol that you'd"made it" in life. You had wealth, you had independence, you had security, you had prestige. Even lower-income white families who did not stand any realistic prospect at ever entering the slave owning classes still benefited from the privilege contemporary racial ideology bestowed on them, which automatically elevated white people above black people. Racial prejudice largely transcended boundaries of class, religion and gender, especially in the 19th century South.

By 1860, almost 4million of the South's 12million people were enslaved African Americans. In half a dozen states, the enslaved population made up well more than 40% of the total. This was an institution that was thoroughly entrenched in Southern society - it was not just a society with slaves, it was a slave society. Whilst comparatively few individuals owned slaves, a sizeable and substantial plurality of white families did; many, many more people benefited from access to slave labour without being slave holders, and many of those aspired themselves to one day be slave holders. So even though ownership in the legal sense was comparatively uncommon, family ownership was exceptionally prevalent and slavery itself was thoroughly embedded in Southern society and culture. Nor were those benefits reserved just for the South, either - there were many in the North who had benefited economically and socially from the fruits of slave labour, and Northern society remained deeply racially prejudiced in its own ways.