Adrian’s answer is on-point, in that you should think of the polycounts (and everything really) in terms of the desired outcome and final project as a whole. Also I’d add that you want to consider the role of the object in-game and the scale/distance it will be used at; if the object will always be in the far distance, then there’s no reason to model, f/e, a high-poly moon in the sky when a simple card/plane will do for the purpose. Your main characters and enemies will have many more details and resolution than a cup on a desk, which is where the design doc comes in, as stated in Adrian’s post.
In the words of Einstein: “It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience. “
Or, as is often paraphrased: “Make it as simple as possible and no simpler.” I’d go one step past that and say “Make it as simple as NECESSARY and no simpler.” You can spend a lot of time optimizing just to do it. Are you going to spend a ton of time ensuring perfect quad retopo work on a background object that won’t even be animated/destructed? You COULD do it, but probably shouldn’t because that time is better spent elsewhere.
