How many muses are there in Greek mythology?

There are nine muses, daughters of Zeus and the Titaness Mnemosyne, goddess of memory. Calliope was the oldest, and Muse of Epics and mother of Orpheus; Erato, Muse of Lyrics; Euterpe, Muse of Music; Thalia, Muse of Comedy; Melpomene, Muse of Tragedy; Urania, Muse of Astronomy; Clio, Muse of History; and Polyhymnia, Muse of Hymns. They were trained by Apollo, the god of music.