A Mutable Item Stack is a special IItemStack object that can only be retrieved from ICTPlayers.
That means all of IItemStack's methods, and those of IIngredients!

It might be required for you to import the package if you encounter any issues, so better be safe than sorry and add the import.
import mods.contenttweaker.MutableItemStack;

A Stack's count is the number of items in that stack!

ZenMethod	Parameter Type	Beschreibung
ZenMethod setCount(count)	Parameter Type int	Beschreibung Sets the stack's count to the count provided
ZenMethod grow(count)	Parameter Type int	Beschreibung Increases the stack's count by the count provided
ZenMethod shrink(count)	Parameter Type int	Beschreibung Decreases the stack's count by the count provided

damage(int amount, ICTPlayer player)

Parameters:

• int amount → The amount of Damage points to give the item
• ICTPlayer player → the player/Entity damaging the item