Currently there is no way to capture and report the number of rows in a report (or across multiple reports).
It is common in dashboard reporting to create a metric (or card visual) to count the number of specific items in a table by simply allowing an application to count the number of unique or distinct rows. For example, I may want to show a metric widget that shows an active count of open tickets (rows in a row report that gathers data across multiple sheets).
I wanted to attempt using an advanced COUNTIF formula in my metadata sheet to populate the metric widget, however I was unable to select any report as a data source to reference a particular range for counting.
I have seen at least 4 or 5 community posts looking for the same request dating back to 2016 and as recently as November of 2022.
I can think of a two possible solution paths from a development perspective:
1) Allow the Metric Widget to reference a specific column in a report and provide functionality within the metric widget settings to show the number of rows in that column and allow the user to toggle a binary option for unique or distinct rows based on the values in the specific column
2) Allow a formula to reference a specific column range where a row report (or summary report) are the available data sources