No, the game doesn't store items that way. It stores each individual copy of an item as a different address, with the value being the item's ID, not quantity. If you have 123 normal flashlights across all your characters, there will be at least 123 addresses with the item ID of the normal flashlight. Because of this, you can't outright change the quantity of an item. You can, however, change the IDs of items you don't want into items you do want (such as changing all your yellow flashlights to purple or yellow moris to ebony). This is what the OP was referring to.
To find IDs:
1. make sure whatever slot the item goes in is empty (i.e. if it's an addon make sure your character's addon slots are empty)
2. do an 8 byte search for "653" (the empty item slot ID that PhantomLegal gave)
3. equip the item you want to find the ID of
4. in your search results several addresses should have changed value to the same number; this is the item's ID
Once you have the IDs:
1. make note of the quantity of the item you don't want
2. search (8 byte) for the ID of that item
3. if multiple characters have that item, you should notice multiple "sets" of addresses that have the same characters at the beginning; each of these sets corresponds to a different character's inventory
4. to find the set that corresponds to your character, find a set that has the same number of addresses as quantity of that item on that character (for example, if you have 23 Tanager Wreaths on Trapper, you should find a set of 23 addresses when you search for the ID of the the Wreaths; each of those addresses is a Tanager Wreath the Trapper has in its inventory)
5. once you've found the right set, add it to the table and change all the values to the item ID of the new item
6. refresh the loadout screen and the items should be changed
Yea but this method don;t add items on my inventory. When i have 50 flashlights and I "add" flashlight on my character still have 50 flashlights not 51.