How to store a number in a item

I havent used MCreator in years, and just coming back to it, how would I store a number in an item?
its unstackable, and it needs to store a number.
i saw in the procedure section, that you can set a custom data number tag to the item, however, I don't know how to add this tag only once. is there some way to see if a tag exists on the item?
or is there a better way to do it?
can I just attach a variable to the item?