UNSC Fleets were large contingents of ships usually assembled and assigned to specific sets of tasks. Fleets of warships were in charge of defending a specific area of UNSC space. UNSC fleets likely differ in size, as Covenant Fleets do, but it would seem that they number around 75 ships though they seldom patrol together, single ships are commonly seen patrolling throughout UNSC space.
UNSC Navy fleets are often broken down further into Battle Groups.
Unlike Covenant fleets, which are dominated by larger vessels such as Cruisers and Carriers, the backbone of the UNSC fleet consists of Frigates and Destroyers. The brass favors this because Covenant ships have better weapons and defenses, thus larger and slower UNSC ships would not have the maneuverability to outrun Covenant weapons, one of few advantages the UNSC has over the Covenant.