I think you're counting some Halycon-class ships in there, not to mention there was only 5 ever named in the books/comics.
UNSC Leviathan --Notable Battles: The Fall of Reach
*Destroyed over Reach*
UNSC Hannibal --Notable Battles: The Fall of Reach
*Destroyed over Reach*
UNSC Prophecy --Notable Battles: Battle for Harvest
*Destroyed in Second Battle of Harvest*
UNSC Say My Name --Notable Battles: Battle for Harvest, Battle for Earth
*Most Likely Destroyed during battle with Prophet of Truth's fleet.*
UNSC Dresden --Notable Battles: None
*Destroyed over Biko*
Go back and count them, 3 are clearly marathons on the command board and another 4 are seen on the vaults of heaven screen.
EDIT: on the second command screen you can see four marathons too.