Durability when player picks up item

I'm trying to make a mod for my modpack.

When you are holding this item in the offhand, it will take 1 damage when the target entity picks up an item.
I want this damage to deal to the item depending on the stack size 1-64 of course... so maybe a variable for the damage?

Reason is, because one of the mods i'm using disables auto pickup. But it has the option to pick up items when a specific item is held in either main or offhand.

I'm new to this, so I could learn something from this!

Thanks for reading - Birbatiel.