I'm not familiar with that report definition, it looks to be an older definition that we're not currently using. Is your version older than 7.6? That looks like an older client report definition, if that's the case then 'number of clients' is typically the number of unique clients (by username & MAC address) that appeared on devices in the selected group / folder during the reporting period.
I suggest looking into the current release (7.7.7), make sure that your hardware supports upgrading to the release as there have been several new features which require additional processing resources (there's a sizing guide under documentation on the support site). In 7.7, we've added more details to the user guide and help texts to help define the reporting fields.