It's a simple enough question.
In past versions of blender, the render tab had performance options where you could specify various things about how blender rendered things. The feature I'm particularly interested in was the one that allowed you to render with a custom number of cores. I like to browse the web and text friends while my scenes render, so I usually set it to render with every core except for one.
But blender 2.8's moved this menu to some obscure location I can't find. (or removed the feature entirely)