Summing numbers from a text dropdown menu

Hi

I have a pdf that contains dropdown menu for type of ID document. In my example, the dropdown option is 'passport'. I want to then create a script that will add my total number of 'crew' if the fields contain the 'passport' dropdown. Ie if I select 'passport' on a row, then automatically the total crew is 1. On next row, if i add 'passport' again, totla crew is now 2. If Passport is not selected on a row then it doesnt add that as a person. The drop down has to be selected to 'Passport' for it to register a person and then add it from each row if selected.