While Jefferson lived, he owned a total of nearly 700 slaves over his lifetime. It is true that after his death, the slaves at Monticello and other Jefferson estates were sold to repay nearly $100,000 in debts.
“Jefferson acquired most of the over six hundred slaves he owned during his life through the natural increase of enslaved families. He acquired approximately 175 slaves through inheritance: about 40 from the estate of his father, Peter Jefferson, in 1764, and 135 from his father-in-law, John Wayles, in 1774.”
http://www.monticello.org/site/plantation-and-slavery/property