About one-third of the population at any one time- it was fairly widespread. The majority of Southerners owned slaves currently or had at one time. Most of the individuals that owned slaves were farmers, with the average holding between 4 and 6 at any one time.

More information here:

http://www.digitalhistory.uh.edu/disp_textbook.cfm?smtID=2&psid=3557