Hey OldAccount29 
…
- For the First Q:
I would pick two, and I can pick 1 in one case, having the AI spawn only around a predefined distance to player to get better performance.
you need to think this way when you design your game, the player will in no way interact with 1000 AI at the same time! otherwise you are building your game for a super genius aliens! silly but its true even those who like open-world games where you have freedom to do whatever you want will get bored if there is no missions or task you have to do to make it challenging. If you ever played games like Mafia, MafiaII or GTA series, the buildings around you are not all accessible and still these games are so successful and have a touching story “for mafia and mafiaII” that made their success and they built the levels based on that story. and even if the buildings are, players will not visit them all and insect them all, it would be a waste of effort for developers. instead they made key building and some additional buildings like a clothes store or gym etc…
even though buildings are not AI, but you get the picture, why spawn 1000 or even 100 AI when player can see only 10 or max 20 AI in his forward direction and fewer around him?
Also for the second pick it is easier because you can make base creature and have little variety for every species. and then identify like hunger/attack animations and can fly or not for each…
2nd Q:
By ignoring graphics I would say mainly CPU and then “not sure but” I think RAM.
3rd Q: I think my first answer gave you a hit about what I am going to say here, It is not a matter of number even if it is well optimized, but why would you waste your client/player resources on things they don’t even see? i.e. care about? like I mentioned there is no need to design each building interior in a street when your player won’t even have the time to get into each of them and only will drive through that street to get to the mission?
And for your Idea it is not as much of complexity issue as it is for the for the creativity, the ease of unreal engine blueprints and behavior trees “the main AI part in UE4” is easier than you think to get along, most behaviors for AI’s will be the same, the diffrence will be in attributes, how much damage per hit or running speed or…
AI uses tick event, and in case you don’t know what tick event is it is event that runs every X frames, ideally every frame but for AI you can define it like every 0.2 second or so. These events what takes your CPU and you would use them on stuff that keeps updating throughout the game, and using it too much on AI is just too expensive for the user.
I hope you get my points and if there are any other questions or points you want to discuss feel free to do so!