The Mass File Upload App offers a straightforward way to determine the number of records to which a file is attached. When selecting the search mode and utilizing features like List View or SOQL (Salesforce Object Query Language), the app displays a message similar to the image below, indicating the total number of records found to upload the file:
This message will provide you with the exact count of records that meet the search criteria.
PS: View Mass File Upload, Salesforce file upload app, on Salesforce AppExchange: https://appexchange.salesforce.com/appxListingDetail?listingId=82cf236b-1ee8-4ea7-aebf-fd0c89fb93bd