Cisco Communications Manager 40,000 plus deployments

The maximum device count for a standard Communications Manager 9.0 Cluster is 40,000 devices, either SCCP or SIP, secured or unsecured devices:
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/9x/callpros.html#wp1287334

If you need more than this you have a couple of options. A "Mega Cluster" is an expansion which increases the device count to 80,000 devices, more information in the above link. Essentially it is additional subscriber nodes, up to 8, allowing for the 80,000 devices. The Mega Cluster can also be used to extend the number of sites from 8 to 16. The Mega Clister contains a maximum of 21 servers, 16 subscribers, 1 publisher, 2 TFTP, 2 MoH.

Another way to increase the number of devices supported above 40,000 would be to run multiple instances of CUCM. This may also be required if you need more than 8 locations.

Design Guidelines for Multisite Deployments

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/9x/dialplan.html#wp1150060

Additional Considerations for Multi-Cluster System

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/9x/dialplan.html#wp1150088