Yes, it's wrong. The inventory is populated in the order you obtain items, so item_POTION could be placed anywhere in the inventory. inventory_qty[item_POTION] reads inventory slot 0 but ANYTHING could be there.
Oh ok, so i cant use draw_text function for this then ?
You can, you just need to read out the correct number of items (using the new inventory_get_qty function I suggested)
Oh ok, then how to draw the qty variable ?
What script i used to draw the number of value for example potion?