The serial number is a unique number assigned by the manufacturer to a product to identify the individual device. Below you can find instructions on how to find the serial number for Mac, Windows and Linux.
Go to the Apple menu, which you'll find in the corner of your screen. Select About this PC, which will open a window where you'll see an overview of your Mac, including it's serial number. You can find more information at Apple support.
Open the command prompt on your computer. You can do this by typing cmd in the search field on the right or by clicking on the Windows home icon. Type the following command: wmic bios get serialnumber and press Enter, your serial number is now displayed in the command prompt. You can find more information at Windows support.
Open the terminal, by pressing Ctrl+Alt+T in Ubuntu, or press Alt+F2, type gnome-terminal, and press Enter. Then type the following command: sudo dmidecode -s system-serial-number and press Enter, your serial number should appear in the terminal.