Gathering all multiples of the same item is called "item stacking." To gather stackable items together, open the "Inventory" menu and press the '+' key on the numeric keypad of the keyboard to select "Sort." If you want to use stacked items individually for item synthesis, you must separate them first. To do this, open the "Item" menu and press the '+' key on the numeric keypad of the keyboard. The cursor will jump to the right-hand side of the screen. Select "Manual." Move the cursor into your item box again, highlight the desired item, and press the confirm button twice. The stacked items will separate into individual items again, allowing you to use them individually.