The getRowCount function retrieves the total number of lines in an API call. You can use it by clicking on "Display number of rows" in the bottom right corner of the Data Query 3 interface.
You can also replace the getData function from your API call...
...with the getRowCount function:
Your call will then retrieve only one piece of information indicating the number of lines in the request.
Approximate count distinct :
The Approximate count distinct function returns the approximate number of different non-null values of a property. This function is based on an algorithm that allows a 2% margin of error.
Applying the Approximate count distinct produces a metric with the value resulting from the function.