Count of Service Account Names will display a count of all accounts used to run a service. This report is useful prior to deleting a service account or changing its password because you can proactively see how many services use a particular account before making any updates. This report drills through to List of Services by Service Account Name, which in turn drills through to Computer Service Details.
Count of Services shows a count of all services. This report drills through to List of Computers by Service, which in turn drills through to Computer Service Details.