creating a variable in data and analysis that provides a count of external data reference use

Okay. So I have a survey that uses a combination of a contact list trigger and authenticator to allow users to use one survey (set up) to populate the contact list with embedded data for another survey (end user). Pretty straightforward stuff. Using this solution during our move to virtual training, as it's much easier to deploy than emails.
One of the questions we ask in the set up survey is the number of anticipated responses. That gets piped over into the embedded data in the contact list for the end user survey. In addition, a unique external data reference value is used to authenticate for that use of the survey. So, let's say one use of the survey expects 20 people to take it. That value and the external data reference they will use are popped into the contact list for the end users.
What the users want is to know the response rate for each use of the end user survey. So, in the example above, say 15 people take it. That would be a response rate of 50%. But I would have to manually calculate that, and that's tedious. So I'm wondering; is there anyway to write in code that would return a count of the number of times an external data reference is used, and save it as embedded data in responses? That way, I could write a simple math formula to calculate response rate somewhere.

Thanks!