The number of realms that your installation requires depends on several factors:
The number of clients to be supported. Too many clients in one realm makes administration more difficult and eventually requires that you split the realm. The primary factors that determine the number of clients that can be supported are as follows:
The amount of SEAM traffic that each client generates
The bandwidth of the physical network
The speed of the hosts
Since each installation will have different limitations, there is no rule for determining the maximum number of clients.
How far apart the clients are. It might make sense to set up several small realms if the clients are in different geographic regions.
The number of hosts that are available to be installed as KDCs. Each realm should have at least two KDC servers (master and slave).