I thought that Muslims believe only in Muhammad as their Prophet. Is this true?

No! Muslims believe in all of the Prophets and Messengers that God sent to mankind from the days of Adam to the days of the Prophet Muhammad. God sent over 124,000 Prophets in the history of mankind. However, God mentioned 25 names in the Qur’ān some of them being: Adam, Enoch, Noah, Abraham, Lot, Ishmael, Isaac, Jacob, Joseph, Job, Moses, Aaron, King David, King Soleman, Jonah, Zachariah, John the Baptist, Jesus and Muhammad.  Muslims believe in all of them and do not differentiate in their missions – all of whom were calling to the worship of one God. Any time any Prophet’s named is mentioned, Muslims say peace be upon him (pbuh).