A fleet represented different things during the different periods. While formal segregations were mostly unknown until the Imperial Sith War era, they became more relevant as war became the chosen answer for disagreement by the forces of the Republic, Galactic Empire and Galactic Alliance.
A 'fleet' sized unit came into existence following the Ruusan Reformations, donating between 27 and 36 vessels from the Judicial Starfleet to a sector, usually vessels of Corellian design, though Dreadnaughts joined the Judicial fleet following the end of the Mandalorian Conflict. The unit formed the basis for the following generations.
A 'fleet' sized unit came in three differing forms under the Empire. The standard sector fleet number 24 Star Destroyers, 1,600 warships and 800 transports, and the standard oversector or priority fleet the equivalent of four such sector fleets. Lastly, the Empire formed squadrons, which consisted of five Star Destroyers usually centered around another larger Star Destroyer. With a quarter of a sector fleet the Empire could bring most worlds to heel.
As the Empire collapsed into a Remnant, the standard sector fleet consisted of three Star Destroyers and various support ships, but the Remnant eventually covalesced so as to have thirteen Star Destroyers a sector, while fielding a oversector fleet to screen the entire Remnant. When the Galactic Empire was restored, it established strategic reserves - numbered fleets - that consisted of initially twenty Star Destroyers of varying types, and later 16 Pellaeon-class Star Destroyers.
The New Republic Defence Force treated fleets differently to the Empire, creating forces which were equivalent in strength to old Imperial Sector fleets, but deployed to entire regions. Five fleets fielded 25 Star Destroyers and 50 Star Cruisers supported by hundreds of warships, split into 25 task forces of one Star Destroyer and two Star Cruisers a piece and seventeen warships - any one of which was capable of overwhelming the then-reduced Imperial sector fleets. Battlegroups consisted of five task forces united against a common foe, and flotilla's consisted of paired task forces. Standard deployments involved task forces and flotillas, though these deployments became rarer following the Battle of Ithor as they were easy prey for Yuuzhan Vong forces.
The entire NRDF before the Yuuzhan Vong War numbered 375 Destroyers and Star Cruisers and 2125 smaller vessels, split between carriers, cruisers, frigates, pickets and corvettes. 2500 ships, supported by captured Super Star Destroyers and newly constructed Star Defenders.
Construction increased as the war picked up, seeing a single Star Destroyer constructed for 250 smaller warships. It was with these numbers that the NRDF swelled to over thirty thousand vessels, and the ratio between a Star Destroyer or cruiser was extended so that 80 smaller vessels to each cruiser. A standard deployment included a Destroyer or Star Cruiser, twenty smaller cruisers and sixty frigates.
More than a quarter of this enlargened NRDF which was destroyed at the Battle of Coruscant. These losses were recovered in the following year, and the NR became the Galactic Alliance, which itself lost most of these vessels surviving a renewed Yuuzhan Vong offensive in 29 ABY.
The Galactic Alliance Defence Force took the principles of the NRDF and expanded upon them, creating twelve twenty Star Destroyer fleet groups to patrol the GA. The First and Sixth were deployed to the Core, the Second to the Yuuzhan Vong invasion corridor, the Third and Fifth to the Outer Rim, the Fourth to the Colonies, the Seventh to Kuat, and the Eighth to Coruscant. The Ninth secured the gap between Yaga Minor and Coruscant, protecting the GA from the Unknown Terrors, and the Tenth fleet onwards protected the Inner Rim, Expansion Region and Mid Rim individually.
Additional fleets were provided by the Hapans, whom provided a pair of fleets, and the Remnant, which fielded four fleets to assist the GA against the Confederation, which took seven fleets when it seceded. Another fleet was fielded by the Wookies, and Hapans keeping a third fleet at home. Three fleets were destroyed at the Battle of Kuat, with subsequent battles at Kashyyyk, Centerpoint and Fondor costing the GA another two fleets before the conflict came to an end.
The GA eventually evolved it's twelve fleets to field 40 Scythe-class battlecruisers and 160 Sabertooths and GA frigates, enough to engage an Imperial fleet. Most of these forces were destroyed in the Sith-Imperial War, a remnant force surviving.
A short lived but remarkably well armed entity, it fielded fleets that corresponded with existing GA principles, fielding two Corellian fleets, one Commenori, three Bothan and a Hutt fleet at its height. These forces were mostly destroyed at the Battle of Kuat, reducing them to a single Corellian and Bothan fleet. Construction never grew significantly enough to create entire units such as these again, the GA fielding its entire fleet at the front rather than retreating. These forces were reduced incredibly by the subsequent battles, until the Confederation surrendered and rejoined the GA at the insistence of the Jedi Coalition and Imperial Remnant.
Another short lived entity, it fielded the equivalent of two fleets, one of which was destroyed in detail at the Battle of Nubia, and the rest which were picked off at the Battle of Utapau, Battle of Xa Fel and the Battle of Bakura, the entity sacrificed by its creators to spite the GA and strengthen the position of the Empire and One Sith.