There was only one ship.
One. Are you sure?
Yes. They called it the Pillar of Autumn.
About this creation
The UNSC Pillar of Autumn (UNSC Cruiser C709) was a Halcyon-class Cruiser serving in the UNSC Navy. Informally, she was simply referred to as the Autumn or abbreviated as the "PoA". The Autumn was retrofitted with advanced weaponry and technology in preparation for a special operation to seize a Covenant warship and subsequently capture the Covenant leadership. Prior to the launch of this mission, the Covenant fleet attacked Reach, during which battle the Autumn was a major participant.
Following the battle of Reach, the Pillar of Autumn jumped to the Threshold system, where she encountered a pursuing Covenant fleet and Installation 04. Her captain, Jacob Keyes, landed the ship on the Halo installation, where her surviving complement mounted a resistance against the Covenant and later the Flood. Following the release of the Flood, the Master Chief detonated the Autumn's fusion reactors in order to destroy Installation 04.
I based this off of Saber-Scorpion's PoA, but the designs are all mine.