I am new to creating game assets; I read that the poly count needs to be somewhere around 64,000. I've created a skyscraper without any modifiers and as basically as I can in order to achieve the shape I want, and the vertex count is 168,000. I am unsure of what steps I need to take in order to reduce the vertex count/ make it usable in Unity. Perhaps I've taken the incorrect approach/ don't understand. I would be grateful for any information.
There are a lot of ways to optimize your model. For example, you can model the single floor, and instance it in the unity. You can use baked normal maps instead of geometry. It's all about the level of details you need in your game. If it's far from the camera you can use a simple textured box. If you walk around but you have no helicopter, you can have detailed first two floors, and less detailed ones above. So it's all up to you. The question is more to do with unity than with blender, so maybe it's better to get some guidelines from the unity gamedevs about you very problem - the best way to build a city in unity.
You need to create a low poly version and a high poly version and then bake the normal maps onto the low poly version.