In order to add more than 3 Build Agents to your TeamCity Server, you should obtain one Build Agent license per each Build Agent you want to add.
Build Agent license is available for $299 per agent to all TeamCity users.
One Build Agent license allows you to connect one additional Build Agent to your TeamCity Server installation, regardless of the Server license you are using (Professional or Enterprise).
A single Build Agent license can not be used to authorize more than one concurrently running Build Agent. For more details, please see the TeamCity License Agreement.
Each new Build Agent license allows for free Build Agent software upgrades with any new major TeamCity version released during the next 12 months.
To continue upgrading your Build Agents software after that period, you'll have to get the Build Agent Upgrade Subscription renewals for each purchased Build Agent license, for $149 per year.
Build Agent Subscription renewals commence from the expiration of your previous subscription period, regardless of when the subscription renewal is purchased.
If you are interested in purchasing a license for five or more Build Agents, please request a price quotation.