Actually, he is talking about DC Directory which was the local x.500 directory option for customers deploying Call Manager 3.x/4.x. I am not sure about the scalability potential of DC Directory. I just wanted to clarify that it wasn't (thread was from 2006) that was being discussed.
The 7,500 figured mentioned for the 7845 refers more to the supported number of phones than users. This is a rough estimation and any large deployment should be planned using the appropriate capacity planning tools.
CUCM uses a server clustering design with the typical maximum number of phones per cluster being 30,000 although Cisco have deployed "mega-clusters" for some customers that exceed this number. Clusters can also be linked together and newer versions of CUCM support cross-cluster extension mobility so the limits are fairly elastic.
The best place to understand CUCM is at the SRND design guide.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.