When I enter my key / serial the installer says invalid number

Make sure you are not leaving any blank spaces before or after the string of characters (this usually happens when you copy and paste).

Copy the Serial and paste it in the Notepad, then copy it from there and paste it in the installer. 

Be aware that you must type the exact same email used on the first install, if still not working, please open a support ticket with your serial number and a screenshot showing the error message.