This threshold is designed to alert you to a potential performance problem in Flow Monitor. As Flow Monitor receives flow information from your network devices, the IP addresses of traffic senders and receivers are recorded in WhatsUp. If your systems communicate with a large variety of hosts, this can cause the table which contains these IP addresses to become large. For example, if you have NetFlow enabled on the public interface of your border firewall, potentially you could receive traffic from a large portion of Internet hosts. This could cause the number of recorded hosts seen in Flow Monitor to be very high.
In some cases, changing the interfaces from which you receive flow information can help (such as only sending flow information from the private side of the firewall instead of the public), but moving to a SQL instance with greater resource capacity can also improve performance. For more information, see the WhatsUp Gold Database Migration and Management Guide on the WhatsUp Gold web site.