OS X: Activity Monitor shows one CPU Usage graph on systems with more than four cores
In Activity Monitor, the CPU Usage and history show a separate graph for each core if the Mac has four processor cores or less, but a single graph if there are more than four cores.
On Macs with more than four cores, a single graph showing aggregate usage of all the CPU cores is shown in the main Activity Monitor window, in the CPU History window, and on the Dock icon when configured to show CPU Usage or CPU History.
Sample CPU Usage tab in Activity Monitor on a Mac that has more than four processor cores
To see the CPU usage for each core individually on systems with more than four cores, use the CPU Usage window or Floating CPU Window. These windows can be viewed by selecting them from Activity Monitor's Window menu. In OS X Lion or later, the CPU History window will also show all cores when there are more than four cores.
Note: The CPU Usage window cannot be displayed if CPU Usage is being shown on the Dock. To stop showing CPU Usage on the Dock, choose View > Dock Icon, then select any choice besides "CPU Usage", such as "Application Icon".
Sample floating CPU Usage window on a Mac with 8 cores running OS X Lion
Note: On Macs with processors that support Hyper-Threading, Activity Monitor will display two graphs for each core because Hyper-Threading allows two threads to run simultaneously on each core. This means that a single graph showing aggregate CPU usage will be displayed on these Macs, if more than two cores (or four virtual cores) are present.
See also: How to identify a Mac Pro (8-core) computer.