The Related Records field in kintone is a useful field that lists other records that match a given condition. The Related Records field though does not state how many records it is listing - which may become hard to count manually when there are many records listed.
This sample code does the counting for you, and displays the number in a Space field of your App.
As shown in the image below, each record will display a different number for the total number of related records.
Refer to the Tutorial page, on how to add scripts to your app.
Note that this sample will only display the number - it will not store and save the total number in a field.