How bad is it using server NPCs?

I’ve seen a lot of people in this forum and outside of it saying that NPCs on the server are bad practice, and I understand why, but I want to know: how much performance-heavy is it? With how many NPCs will the server start to lag?

(Assuming OOP is being used correctly to reduce the performance impact of each NPC)

I have never once heard about running npcs on the client? Can u provide me with source

Edit: my mind went to the npcs that people test combat on and ect haha. For the moving NPCs you should do it on the client. Whenever the npcs add up the server starts to lag bc of the animations

Well my answer was just ready lol

Please reread my topic, I want to know: