It depends on EVERYTHING that is going on at the time, and what software is installed, AND what game you are playing.
It changes from second to second. Your system will normally, even when "idle" go from using no cores to using one or two, to sometimes using them all, depending on what is going on. The system itself may at any given time be using cores to run virus and malware scans, creating system restore backup points, file system indexing, file structure optimization (Defrag, TRIM), checking for system and security updates, and fifty other potential processes as needed. And that's without even factoring in usage for installed applications that might themselves at any given time be checking for it's own updates or running processes in the background.
Unless I'm not understanding what it is you are asking. What, specifically, is it that you are unsure about?