This is the number of boards or panels you require. If your design is a single board, it is the number of single PCB boards.
If your design is a panel with several boards on it, it is the number of your panels.
You can choose the number available on our website only, self-defined numbers like 12pcs or 18pcs are not supported. If the quantity that you need is more than the largest one on the website, you can contact our support for help.