Set-Up and Configuration Overview Report
Navigation: Reports > Setup and Configuration Overview (under Usage Tracking section) > look at the first line: "# of Administrators: 142."
Setup and Configuration Overview is the name of the report.
The reason was that our administrators (those whose role have "admin" in the role name) are only 52 (active and inactive).
I raised a service request and this was Oracle's response:
The "# of Administrators" metric is calculated by a SQL MAX() function, which in this case is MAX(# of Administrators).
The column currently returns a value of 162 which is equal to the total number of Administrators you have had in the system throughout time.