How does FileAudit count users?

The number of users in FileAudit is calculated by the number of users accessing the files that are being audited. These are the users you see in the access events as shown below:

To help you determine the number of users accessing files, you can use the diagnositic.txt file that can be generated by hitting F12 in the FileAudit console.
If you have version 5.5.0.24 or newer, you can see the number of users in the last 30 days. If you have version 6.0.0.44 or newer, you can also see for the last 30 days as shown below: