Margaret Mitchell had published 0 novels before Gone with the Wind. It was her first novel published during her lifetime, appearing in 1936.
Mitchell had written stories and manuscripts as a child and teenager, including the novella Lost Laysen and a long, unfinished work sometimes called The Big Four. Those early writings were not published novels before Gone with the Wind. Another manuscript, Ropa Carmagin, was submitted to Macmillan but rejected as too short for book publication.
Mitchell began writing Gone with the Wind in 1926 while recovering from an ankle injury. The manuscript grew into a sweeping historical novel about Scarlett O’Hara, the American Civil War, and Reconstruction-era Georgia. Macmillan published it on June 30, 1936.
The book became an enormous success, winning the Pulitzer Prize for Fiction in 1937 and inspiring the famous 1939 film. Lost Laysen was published posthumously in 1996, so it does not change the answer to this question about works published before Gone with the Wind.