well, whole question in subject.

im searchign for files where it can be changed

Whoops, AI modding is not officially available yet and that's why the link doesn't work, my bad.

Unfortunately the AI's decisions are done via code in a specific DLL and stuff, and thus cannot be accessed via XML.

However if you want players and AIs to be unable to build more than one large ship, you can add a prerequisite on the large hulls which make them unavailable for construction once the empire already owns one.

Simply override HullDefinitions such as HullLarge01Cravers or HullLarge01Sophons, by copying their content into your mod and adding the prerequisites to their list.

This prerequisite will restrict the construction of Large ships to one per empire, player included:

<PathPrerequisite Flags="Prerequisite,Discard" Inverted="true">../ClassEmpire//ClassShip,ShipSizeLarge</PathPrerequisite>

While these ones will restrict it only for AIs:

<PathPrerequisite Flags="Prerequisite,Discard" Inverted="true">../ClassEmpire,GameDifficultyAINewbie//ClassShip,ShipSizeLarge</PathPrerequisite>
<PathPrerequisite Flags="Prerequisite,Discard" Inverted="true">../ClassEmpire,GameDifficultyAIEasy//ClassShip,ShipSizeLarge</PathPrerequisite>
<PathPrerequisite Flags="Prerequisite,Discard" Inverted="true">../ClassEmpire,GameDifficultyAINormal//ClassShip,ShipSizeLarge</PathPrerequisite>
<PathPrerequisite Flags="Prerequisite,Discard" Inverted="true">../ClassEmpire,GameDifficultyAIHard//ClassShip,ShipSizeLarge</PathPrerequisite>
<PathPrerequisite Flags="Prerequisite,Discard" Inverted="true">../ClassEmpire,GameDifficultyAISerious//ClassShip,ShipSizeLarge</PathPrerequisite>
<PathPrerequisite Flags="Prerequisite,Discard" Inverted="true">../ClassEmpire,GameDifficultyAIImpossible//ClassShip,ShipSizeLarge</PathPrerequisite>
<PathPrerequisite Flags="Prerequisite,Discard" Inverted="true">../ClassEmpire,GameDifficultyAIEndless//ClassShip,ShipSizeLarge</PathPrerequisite>

You can also replace PathPrerequisites with InterpreterPrerequisites to check for a specific amount, instead of "one or zero":

<InterpreterPrerequisite Flags="Prerequisite,Discard" Inverted="true">Count(Context,@'../ClassEmpire//ClassShip,ShipSizeLarge') ge 1</InterpreterPrerequisite>