On acorn use
	If [acorn stack] more than or equal to 10
		For each 10 in [acorn stack]
			[acorn stack] -= 10
			Give bento box
		End for loop
	Else
		Display message "You need a stack of 10 or more acorns to make a Bento Box"
	End if
End function

If you're unable to reference a specific stack of items, do something like this.

On acorn use
	Create int variable [acorns]
	for each item in inventory
		If [item] equals item:Acorn
			Add 1 to [acorns]
		End if
	End for loop

	If [acorns] more than or equal to 10
		For each 10 in [acorns]
			[acorns] -= 10
			Give bento box
		End for loop
	Else
		Display message "You need 10 acorns to make a Bento Box"
	End if
End function