Adding variable in response of call

Hi guys! When running a query of all records, I'd like to add a variable that counts the number of sub-lists of things I get via doing an inner join.

Example:
List of Makers, inner join with Cars.
We end up with the list of cars by makers.
Now in the API call, I want to sort the list of makers based on the numbers of cars each Maker has. Example: Ford 1000, vs Hyundai 500, vs Tesla 10.

Updating a variable that already exists in a call is automatically called in the APi. However creating a variable isn't part of the response call, and I didn't find myself the way to add it within the response call to be part of the list rather than at the end, as an aggregated variable (1510 rather than within each maker object)

Thanks for the help!