Answer: one
Only Karl Thomas is shown in the movie, and the child never says a word! In truth, Constanze gave birth to a son, Franz Xaver Wolfgang, in July 1791, 5 months before Mozart died. This pregnancy/child was never shown or mentioned in the movie! (There has been some speculation about the paternity of Franz Xaver.) Of the six children, Karl Thomas and Franz Xaver Wolfgang were the only two who survived into adulthood. Salieri actually taught Karl Thomas as a composition student after Mozart's death. He taught him for free.