[Question] Get current running total of items clicked

How to use this repo to get current number of items selected in dropdown and disable, say if 20 threshold is exceeded?

i.e Modal defaults to blank then as user clicks will show:

Selected 1 of 20 selected
Selected 2 of 20 selected
Selected 20 of 20 selected etc

one more click:

Cannot exceed 20 and also disables click until unless than threshold