Added a placeholder for PlaceHolderAPI
%itemedit_amount_<{itemid}>_[{slot}]_[{player}]%
shows how many itemid player has on slot
<{itemid}> for item id on serveritem
[{slot}] for the slot where the item should be counted, by default inventory
Slot types: inventory, equip, inventoryandequip, hand, offhand, head, chest, legs, feet
[{player}] for the player, by default self

use example:
get the number of item with id my_item_id in your hand
%itemedit_amount_{my_item_id}_{hand}%