A fleet is a large grouping of starships travelling together or assigned to particular duty.
The term fleet can be applied to any significant gathering of starships as well as to refer to the totality of a power's space based forces. Military powers with large numbers of vessels often organise their forces into specific fleets commonly referred to by a numerical designation.
During wartime the Breen organize their forces in brot'kar, which are fleets based on the state that contributed the vessels. As such the size of brot'kars vary as more powerful states can field hundreds of vessels while the lesser states might only be able to contribute a dozen. (The Dominion War Sourcebook: The Fires of Armageddon)
Cardassian Union orders operated fleets, called shodaro of 150 - 300 starships. A shodar would be commanded by a jagul or gul. (The Dominion War Sourcebook: The Fires of Armageddon)
The Romulan Star Navy's Avara (century) designation was for their fleet units of around one hundred vessels. (The Dominion War Sourcebook: The Fires of Armageddon)
Starfleet operated numerous numerical fleets made up of hundreds of starships. These fleets were usually commanded by a high ranking flag officer, such as an Admiral or Fleet Admiral. These fleets were usually based around several tactical wings. (The Dominion War Sourcebook: The Fires of Armageddon)