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 Kerberos traffic that each client generates
The bandwidth of the physical network
The speed of the hosts
Because each installation will have different limitations, no rule exists for determining the maximum number of clients.
How far apart the clients are. Setting up several small realms might make sense 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, one master server and one slave server.
Alignment of Kerberos realms with administrative domains is recommended. Note that a Kerberos V realm can span multiple sub-domains of the DNS domain to which the realm corresponds.