There is no built-in way to restrict the number of connections per Connector, but you can easily achieve this.
You should handle the ConnectionManipulationStarted/Completed event and in the event arguments, you can check something like the following and mark the event as handled - this will prevent the user from creating the connection:
if (diagram.GetOutgoingConnectionsForShape(e.Shape).Count(c => c.SourceConnectorResult == e.Connector) > 2)
e.Handled = true;
Please let us know if this is working for you.
the Telerik team