Here are a couple of ways to find out the number of CPU cores when using Linux Mint / Ubuntu :
The graphical way :
1. Go to System > Administration > System Information.
2. Drill down to “Devices” and select “Processor”. The right pane shows the number of CPU cores for the given system.
The command line way :
1. Open Terminal and install a tool named “htop” :
2. After installing, run htop by typing :
Note the first few lines. They show numbered rows along with a percentage. These numbered rows indicate the total number of CPU cores present ( 2 rows numbered 1 and 2 here meaning 2 CPU cores) along with their % utilization.
To exit htop, either hit F10 or “q”.
By the way, htop can be very useful when checking out usage of CPU cores  as well as memory when using a Ubuntu server. Linux command line as usual rocks. 🙂
Cheers.