You can either have a global variable that adds 1 every time you collect an item. Then say when global variable is 20, play the sound. Or you can count how many items are in the level, which is items.count and set this to a variable, when the variable is 0 that means no items remaining and you can play the sound.
I did that first option and when I collect 20 that sound plays endlessly even when I set "not looping". How to make it play only once?