This displays the number of items on the green wire (supplied to the * (multiply) "Arithmetic Combinator") for the item selected in the "Constant Combinator". The Constant Combinator needs to be set to 1 (i.e. default).
This blueprint is kinda complicated but you can add more numbers to the display by just by copying the 2 leftmost combinator stack and pasting while overlapping, pre-existing combinators.