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 XXXX with 07D9 give you love potion
Couple things you should know on the codes for editing items.
The codes effect the first item in what ever window you have open. If you have your storage open and on the fridge page, it will effect the first item in your fridge. Same if you have your tool box, armory, storage box, so on open.
Second, Do not attempt to change the item from one category to another. Like if you change a potion into a sword, it messes that item up.
Since I am on a new account on the forum, I can not post a link, but if you search for Rune Factory 5 1.0.1, you should find it.
Good place to find codes, examples, and advice.