I know this is an unpopular view, but I think it is important to recognize that the Enterprise and Starfleet weren't intended to be the military of the Federation at the beginning of TOS (which should be read as not the only military).
One can look at Starfleet as playing the same role that the United States Coast Guard does today. In Kirk's original log entry for WNMHGB...
Enterprise Log, Captain James Kirk commanding.
We are leaving that vast cloud of stars and planets which we call our galaxy. Behind us, Earth, Mars, Venus... even our Sun, are specks of dust. A question... what is out there in the black void beyond?
Until now our mission has been that of space law regulation, contact with Earth colonies and investigation of alien life. But now, a new task... a probe out into where no man has gone before.
Compare that to the mission statement of the United States Coast Guard...
For over two centuries the U.S. Coast Guard has safeguarded our Nation’s maritime interests in the heartland, in the ports, at sea, and around the globe. We protect the maritime economy and the environment, we defend our maritime borders, and we save those in peril. This history has forged our character and purpose as America’s Maritime Guardian — Always Ready for all hazards and all threats.
And their own self description...
The U.S. Coast Guard is one of the five armed forces of the United States and the only military organization within the Department of Homeland Security. Since 1790 the Coast Guard has safeguarded our Nation's maritime interests and environment around the world. The Coast Guard is an adaptable, responsive military force of maritime professionals whose broad legal authorities, capable assets, geographic diversity and expansive partnerships provide a persistent presence along our rivers, in the ports, littoral regions and on the high seas. Coast Guard presence and impact is local, regional, national and international. These attributes make the Coast Guard a unique instrument of maritime safety, security and environmental stewardship.
Starships are far more like Coast Guard Cutters in their design and operation than like military vessels of the US Navy. In fact, I'd say that the original Enterprise (in design and intent) is more like today's USCGC Healy (WAGB-20) than any aircraft carrier. And by the same comparison, the United Earth Space Probe Agency is a lot like our National Science Foundation or National Oceanic and Atmospheric Administration. I would suggest watching these two videos on the Healy (here and here) to get a feel for her type of missions.
Why the change from intent to execution?
Well, to start, it was because compelling war stories are easier to turn out than science/research stories. And you can see this in the first season with the use of the recycled war story of Balance of Terror (from The Enemy Below). And the viewers (later fans) loved those stories.
This is where fans started injecting the tropes like the Enterprise was the flagship of the Federation or the most famous starship (in universe), or the Mary Sue turn that the characters (and the Enterprise herself) would take as time went on.
A great example of this change in the look and feel of the Enterprise as she appeared in Star Trek: The Motion Picture. What we saw was the equivalent of this...
The Enterprise started out with no weapons (or minimal weapons) to becoming this weapons platform where that was her most distinctive feature.
While fans some times say that Star Trek became like Star Wars after Star Wars... personally, fans were pushing to make Star Trek into a star wars while it was still on the air.
Honestly, the writers should have made a separate Federation Naval Force early on to help distinguish the Starfleet and it's part in the military hierarchy.
But (unfortunately) that starship sailed early on in Trek history.
Never was the Enterprise depicted as a battleship. At best a cruiser in the old sense of the word. A vessel large enough to act independently and was flexible in its mission types. In the days of sailing this would be a sloop or a frigate.
Cruiser is a underused term in modern navies, having been replaced by destroyers for the most part.
Well, I'm sorry to further muddy the waters, but this needs to be pointed out:
I scanned in Page 115 from the novelization of TMP because I'm tired of typing it in like I have in the past.
The point I'm making is that
Ithekro and
Shaw are both right and both wrong. The middle paragraph in the above scan of Page 115 should make it clear that Mr. Roddenberry straddled both points of view. (NOTE: Page 115 reflects the scene in the movie when Spock's courier-craft was first approaching the refit Enterprise.)
Folks in this thread have repeatedly referred to non-canon material, such as office memos among the shows writers and producers, to make a point about what "There are only twelve like it in the fleet" meant. There are very powerful arguments being raised here to justify different worldviews on what Kirk's boast to Capt. Christopher meant. There has also been no shortage of people talking over each other. Other TV series, gaming, fan-fiction, aftermarket spin-offs and adaptations, and a whole host of other phenomena have evolved since the 1960s. A whole host of worldviews of what
Star Trek is and what it represents evolved along with all of this.
When I spoke about CDST (Contradictions and Discontinuity in
Star Trek), I wasn't kidding. TOS, and to a certain extent, the entire
Trek franchise, was often "specifically vague", meaning that it was often attempting to be flexible enough to appeal to more than one point-of-view. The important thing to distinguish is that Trek speaks with more than one voice. As an example, Gene L. Coon, noted TOS writer and producer, has been said to have written or re-written TOS episodes that favored American involvement in Viet Nam ("A Private Little War") and also against involvement. In essence, Mr. Roddenberry and Mr. Coon produced a series (in its first two years) that provided a platform for an unusually diverse smorgasbord of stories and points-of-view. It should be a small surprise that there would be some conceptual conflict.
But what does all of this mean, going back to the O.P. in this thread? What does "twelve like it in the fleet" mean? Some folks here will adamantly swear this means there were only 12
Constitution-class starships in the United Federation of Planets during the first full year of TOS. But the "twelve like it" quote has itself been blown out of proportion, since Kirk never explicitly mentions the
Constitution-class or any classification of space vessel.
The controversy of the starship-class status of NCC-1017
Constellation is another example. Again, no character ever explicitly says what class of spacecraft
Constellation is a part of. We can assume that, in its final form,
Constellation is a
Constitution-class starship. But the Federation, as depicted in TOS, is about 100 years old. That leaves plenty of time and opportunity for Starfleet to have employed multiple earlier "heavy cruiser" designs prior to
Constitution being commissioned. Since, in TOS alone, we have seen the ship obviously undergo refitting/rejuvenation of her hull and internals, and TMP showed us additionally that a starship can be "almost entirely" rebuilt, it shouldn't be too much of a jump to assuming that
Constellation could easily have started service in an earlier class of cruiser, possibly predating
Constitution by many years. It should be obvious that the Federation itself has been around quite a few years longer than the
Constitution-class of starships.
So, even if we assume that "twelve like it" did mean
Constitution-class starships, Kirk's boast likely meant the exclusive number of
Constitution-class vessels that were built all-new. These would be apart from old cruiser designs that would come due to be refit/rebuilt, as a cruiser of much older, less advanced design (
Constellation?
Republic?) would be.
This certainly makes more sense than assuming that either (1: the
Constitution-class of starships is as old as the Federation, or (2: the Federation only started launching starships-of-the-line in the mid-23rd century, having no earlier heavy cruiser designs prior to that time.