TeamCity grants licenses for common users automatically when they log in to it for the first time. One TeamCity license is always reserved for the administrator who performs its initial setup and configuration.
If the number of licenses is limited and there are no licenses to grant, TeamCity administrator can pass the license from one user to another.
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The number of licenses available is shown on Licenses page of the Administration space. Click view all users.
All entered in the server license keys are stored in <TeamCity Data Directory> /system/license.keys file. You can manually edit this file and then restart server to apply your changes.