If I understand you correctly you may get the amount of items in your inventory by using the “lenght” node on your items array. If you want to store the amount of every single item you may implement a counter or use a struct instead of an array by changing the type of the variable. A struct is like an array with one unique entry and one not-unique field belongung to it.This would allow you to have one unique entry for each item and one counter for them.
To update the logic you could call any function at the end of your adding/removing function.