There seems to be a contradiction between this Wiki page, other wiki pages ('Battle of Derne' and 'Presley O'Bannon') as well as the new book, 'Thomas Jefferson and the Tripoli Pirates.' 

 While this article states that 500 marines were present during the long march and attack at Derne, the other three sources claim a much smaller number.  The 'Battle of Derne' WIki page uses the phrases a "squad of Marines" and "A small detachment of U.S. Marines..."  The 'Presley O'Bannon' Wiki page says that "he commanded a detachment of seven Marines and two Navy midshipmen in General William Eaton's small army during the Tripoli campaign of the First Barbary War."  The above mentioned book agrees with the O'Bannon page.