As far as I know, you can not change the quantity, only what the item is and the quality. And yes, you have to edit the sheet a little bit.
For example
[slot 1]
580F0000 060EACA0
580F1000 00000020
780F0000 00000010
640F0000 00000000 0000XXXX <-------- This is what you would edit. Replacing...