Bach married his second cousin Maria Barbara Bach in 1707. They had seven children, four of whom survived to adulthood:
Catharina Dorothea (28 December 1708 - 14 January 1774).
Wilhelm Friedemann (22 November 1710 - 1 July 1784).
Carl Philipp Emanuel (8 March 1714 - 14 December 1788).
Johann Gottfried Bernhard (11 May 1715 - 27 May 1739).
Maria died in 1720, and Bach married Anna Magdalena Wickle in 1721. They had a further thirteen children, six of whom survived to adulthood:
Gottfried Heinrich (1724-63)
Elisabeth Juliana Friederica, called "Lieschen" (1726-81)
Johann Christoph Friedrich, the 'Bückeburg' Bach (1732-95)
Johann Christian, the 'London' Bach (1735-82)
Johanna Carolina (1737-81)
Regina Susanna (1742-1809)
Total: 20
http://wiki.answers.com/Q/How_many_children_did_J_S_Bach_have