The Quantity Prompt is the pop-up that displays each time you add any item from the catalog into a project spreadsheet. On the prompt you can edit the line item number, quantity, discount and markup. There is also a box that can be checked that says "Don't show again".
If you have turned the quantity prompt off you can easily turn it back on using the Quantity Prompt button found on the toolbar on top of a spreadsheet. This button will toggle the Quantity Prompt On and Off when needed and by default this feature is tuned On. You can also use the Hotkeys of CTRL+Q to enable or disable this feature.