How many Offline Address Books can I hold on a dedicated OAB server?
Microsoft recommends a maximum of 1000. It is possible to hold more than 1000, but hosters who choose to do so should monitor their Active Directory performance closely. Details are in http://support.microsoft.com/kb/945629/en-us. Hosters expecting to have many thousand hosted organisations should deploy web-based OAB distribution in HMC 4.5.