I cannot really comment on that since I never play to that stage in the game.
But wouldn't making only one ship mean that you get flagged "ripe for conquest" and get ganged up upon by the AIs?
I wish they would.
But no, my endgame ships are worth between several hundreds to thousands of military power each, so having one fleet is enough to tower over the rest.
That is due to the limited blueprints the AI has access too. Would be a different story if the AI actually took advantage of all the miniturization tech, augments and repair components like players do.
Hence why the AI's ships are never really a threat unless you have no military. They tend to opt for the first two specializations (reduce cost or more power/range). Very rarely do I ever see anyone with a miniaturization specialization. Nevertheless, even if they did took advantage of mass reduction techs and support components and made better blueprints, they would still only be a threat to each other.
Firstly, they tend to be too busy fighting with each other to develop many tech trees to begin with. One problem with the AI (at least in my opinion), is that they are STILL too quick to go to war. A few dozen turns building out, and then most of their production goes toward military. This means they are going to be running ships with one type of weapon and one type of defense most of the time, making them extremely exploitable in playing paper-scissors-rock.
Secondly, they do not make good fleets when their ships hit huge size. As far as my observations go, their huge ships and carriers tend to be solo, making them easy pickings. They stop making good fleets at large. If they at least banded together their carriers, they would be more trouble to deal with.
Finally, the third nail in their coffin is the poor colony planning. I noticed significant improvements to the AI since earlier versions, but this still does not hold a candle to the player's planning. So even if they did took advantage of mass reduction tech and made competent ships, they would not be able to afford to make the darn thing. While I, on the other hand, am rolling out ships worth 20K military production in a few turns powered by a level 20+ Hyperion Shrinker. Speaking of high level improvements, my level 18+ Hyperion Logistics means that my fleet will always be bigger too.