Starfleet has a problem:
Space is Big . As borders expand in a linear sense, the amount of space Starfleet needs to patrol, and control grows geometrically.
The upshot of this is that there are never enough trained crews and ships to do everything that Admirals would like.
Starfleet is constrained by demanding a high level of competence and reliability from it's people. This problem appeared early on in Starfleet and all attempts to keep pace have failed. As Starfleet grows, the job it must do grows ever faster.
Early on, A Starfleet composed of loaned officers from the various member worlds star services suffered from problems. Different training and cultures impeded Andorian and Tellarite Officers from cooperating with horrible results.
The decision was taken to centralize Starfleet training in San Fracisco, in order to create unified training and to encourage a "Starfleet Culture" to form.
In ST-OM, in order to become a commissioned officer, the candidate must train at least six months in San Fracisco, even if his technical qualifications are perfect.
This has lead to a bad bottleneck.
Some Solutions Edit
More enlisted people. Enlisted people in Starfleet are more narrowly trained, and have a shorter committment of time, but are easier to recruit and train.
Warrant Officers: In some areas, Admirals have taken to using "Warrant Officer" ranks to promote officer candidates who can't get back and forth to San Fracisco easily.
Exchange Officers: Starfleet allows exchange officers from allied and subsidiary forces. Some admirals wind up having a signifigant portion of their commands staffed by exchange officers.
Technology: Dr Soong and his Android project were watched closely by Starfleet, hoping that his androids could be successful assistants to Starfleet. Data has, indeed been a valuable member of the fleet, but ethical concerns prevent mass manufacture of Soong type androids.
Holographic assistants are also used, but these have been revealed to hgave their own sets of etrhical issues which makes wide spread use impossible.
Civilian Contractors: Starfleet Commanders on the frontier are allowed to hire civilian contractors and assistants to fill desirable roles. Mercenaries and old space dogs can find berths on Starfleet ships, assuming they're willing to respect the Starfleet chain of command and regulations.
In time, Starfleet will be forced to open up Satellite campuses of Starfleet Academy to allow more officer candidates to be trained.
The Bright Side for The Federation:Edit
Both the Klingons and the Romulans have the same issue but sharper limits. In Klingon space, Usually only members of the Warrior Caste can serve as officers. Enlisted Klingons may come from any caste. The New Klingons will accept non Klingon enlisted people. But the Warriors must rule.
In Romulan Space, none but the Romulans are considered trust worthy. Although the Empire has Orion, Human and Reman inhabitants (As well as others) Service in the Romulan Imperial Star Fleet is only open to Romulans. The lesser races may serve as soldiers (canon fodder) and as civilian contractors (repair men and janitors) Only true Romulans get Starships.
This imposes a much sharper brake on expansion than the Federation suffers.