I think I have it now. At least, how to add the quantity number. After watching the part of the video I needed most, I set the blueprint like so.
But this was just a test, so I’ll know what to do when the player buys an object or picks it up or receives it as a prize. Though I had to set the Quantity’s Default Value to -1, since the BP skips from 0 to 2. But is there a better way to fix that, if the Quantity skips to 2 when its Default Value is 0, even though the BP is supposed to add 1 for each time the trigger box is touched?