There are two methods to address this issue.
1. Look at the system log file, accessed from the user profile. Just go to your profile settings and there you will have link to the system log file. In the log file search for the string “The number of authorized licenses has been exceeded. Please contact your system administrator.”
Note: This string will appear with the user who is attempting to login, just count the number of instances on this log using your browser simple search tool.
2. In the Admin->Users you can open the user details information and set the user to be the Account Admin of the system. That will send a daily report to this user with sum of all the failed login onto the system.