Professional Server license grants you the right to:
If you require additional build agents or build configurations, you should obtain additional Build Agent license for $299. Each additional Build Agent license connects 1 additional build agent and adds 10 more build configurations to your Professional Server. Technical support is not available for the Professional Server.
To qualify for unlimited number of build configurations and technical support, you should purchase Enterprise Server License.
For more details, please see the TeamCity License Agreement.