Count number of times values selected in a dropdown.

Hello, 
First timer here, trying to automate an incident sheet for our fire co.
I'm trying to create scripts to count the number of members who responded on a certain vehicle.
We have a list of 40 people and there is a drop down next to their name. the dropdown has the vehicles list. 
EX: if we have 6 members respond on 52-12 , I would like the script to count the number of times 52-12 was selected in the drop-down and populate that on another field for a total. Simplifying things for the guys.