The game will use around 6 cores, but cores are not used equally because there are many different workloads being run. Many tasks have to be done on the main thread in Unity, which consistently takes the most amount of time per frame. The same with the graphics thread. Beyond that, other threaded tasks like mesh creation, collision mesh baking and AI pathing are on demand as needed. I have a 12 core CPU, but it certainly does not need or use all of those cores. Extra cores are nice to have, since the OS and other programs can run without using cores the game is using.