I just want to lower AI officer numbers, the current number is 1. Does making it 0.5 make them %50 less officers or it affects only the over the limit ones? Like the limit is 10 and having half mercenary officer only affects after 10?
If affects the over-the-limit ones, yeah.
You can increase baseCombatShipsPerOfficer - it does pretty much what it says, the average number of ships per ship with an officer.
What about remnant ships, how do I lower AI core officers? What does "baseFPPerOfficer" do?
For AI cores, it's slightly different - baseFPPerAICore - which determines the quantity of AI cores based on the "fleet points" value of the ship, which is not a player-visible stats. It's kind of like the deployment cost, but lower than that for larger ships. Raising that number will reduce the number of AI cores in Remnant fleets.
baseFPPerOfficer is commented out and won't do anything.
Keep in mind that some fleets are scripted to have a large number of officers or AI cores - for example, the high-level Remnant bounties from contacts - and they will not be affected by these kinds of settings.