In AI War, most ships have a Cap. It is displayed in bright green in the upper right corner of their ID card, on the economical line, after metal cost, energy cost and build time.
A ship cap is the maximum number of identical ships a player can control at the same time. When one is destroyed, one can be immediately recreated, paying its Metal cost. Most fleet ships have a cap of 96 and most Starships have a cap of 2.
Cap per Mark
There is one ship cap per Mark level. That means a planet can have a full cap of MkI units and a full cap of MkII of the same kind of unit. This has the effect of never making useless lower mark units: they can always add their modest cap to the fight, even if they are outgunned and if the players have better. Only Turrets might replace lower mark turrets because they have a significative yet constant Energy cost: a high mark turret cost as many energy as a low mark turret of the same kind, and as energy might be scarce, it might be interesting to save some by building only the highest mark possible.
By default, a cap is galactic, which means that all ships in the galaxy are counted for that cap. However, most Turrets have a per-planet cap, which means up to a full cap of them can be built on each planet. For example, the MkI Needler Turret has a planetary cap of 48, which means a player can build 48 needler turrets on a planet and 48 other needler turrets on another planet. However, their Energy cost might prevent the construction of all available turrets on every human-controlled planets.
Cap through Mark
Most units have a constant cap through their mark levels. However, some rare units have a higher or lower cap at different marks. For example, the Heavy Beam Cannon has a lower cap at higher mark (and a cap of 1 at MkIV), and the Neinzul Scapegoat has higher and higher caps (from 48 at MkI to 240 at MkV).
Some units can be granted off cap. For example, by opening a Zenith Reserve, a player can control more of a unit type than its cap. There is no problem to this, only that if said units are destroyed, the player won't be able to recreate them. For example, while a Zenith Power Generator have a cap of 1, players can capture as many of them as they can find. However, they can only build one (by buying it from the Zenith Trade Ship) if they don't already control one. This is why it's better to at least begin the construction of the trader's Zenith Power Generator before capturing an AI controlled one. This is true for all other buildable/capturable units.
While Human players can give units to one another, they can't give units that would exceed the receiving player's cap.
AI and Cap
AI players are not limited by ship caps. This may lead to apparently unfair situations where they take advantage of a large number of low-cap units, while being fair on the total power.