Whenever you need to enter a quantity in Linnworks Mobile, you have a choice of keying in the quantity using a built in numpad or swiping to the left will present with tally screen, where you can enter quantity in a box, number of boxes and add them together. 

Another option is to scan barcodes of individual items to add them up together. You can can get to this screen by swiping left twice from Quantity Entry numpad. Worth noting you can scan SKU, barcode or any product identifier.