Fleet Organization
Upon the conclusion of the Romulan War, the various fleets of the victors were combined into one sombined Starfleet, in service to the fledgling United Federation of Planets. Due to the disparate units now under control and the need to unite the personnel, tactics and organizations, the concept of multiple fleets was put aside for the foreseeable future. Instead, Starfleet Command provided control, administration and orders from its new headquarters in San Francisco, on Earth.
When one vessel just couldn't do the job, however, the local Starfleet authority -or Starfleet command, if it was within communications range- would detail the necessary units from whatever their individual current assignments might be. This task force model allowed the UFP to show the flag or exert just the right amount of force, when necessary. With the conclusion of the emergency, the task force would be broken up and the units would return to their mission areas.
This concept worked well enough for the next century, give or take a few decades, but it would sometimes suffer from a lack of available starships. Results could vary from unqualified successes to unmitigated disasters, but Starfleet did what it could do with what resources it controlled. However, lack of available starships as a reason for situational difficulties ceased, suddenly, to be a problem in 2341, as replicator technology was realized. Now, starships could be mass produced, with only the design and assembly phases taking up the significant portion of the production process.
However, with the advent of scores of new starships warping around the Federation and the frontiers, Starfleet orgnization had reached a crisis point. As long as the UFP remained at peace, the organizational snarls and spotty communications could be overlooked in the name of Starfleet captains' traditional independence and initiative. But, the Cardassian and Tholian Wars (2347 - 2366 and 2353 - 2360, respectively) strained the system past the breaking point. Certainly, the outcomes of certain battles -and possibly the length of time of the conflicts themselves- could have been affected with more precise administration.
At Starfleet Command, Admiral Taneko brought his Bolian genius for organization to the problem between 2348 and 2350 and, based on the lessons of both wars, developed the new fleet model that lasted, and proved itself, through the Dominion War and beyond.
In Taneko's fleet model, Starfleet organizes itself into fleets comprising around 150 to 250 ships each, under the command of a Rear Admiral (Upper Half) or Vice Admiral [see Starfleet Command:Ranks for details]. The 'flagship' of a fleet is simply the ship best suited for command-and-control functions in an emergency (as determind by the admiral) and, sometimes, is pre-selected and maintained in a ready state at the admiral's assigned starbase.
Each fleet serves either a tactical purpose (such as to protect Vulcan) or a strategic one (such as ready reserve). Most tactical fleets are 'standing' fleets, which retain the same mission throughout their existance. Most strategic fleets are either 'mobile' fleets, sent to any crisis area or placed on deep-space patrol and exploration duty, or 'supporting' fleets kept ready for specific contingencies. The deployment of a mobile fleet essentially follows the older task force model, only with better communications and more ships. Most mobile fleets keep their starships in the same broad area for faster concentration and coordination in case of emergencies.
On the eve of the Dominion War, these fleets numbered 27, in total, which itself was a significant increase over the last ten years, due to the re-emergence of the Romulans as an active threat and as a response to the Borg incursion of2366. During the war, the number of fleets grew to 44 in number, although the hard fighting left many of them severely understrength.