The link below states that a maximum of 7500 users are supported in a Unity Domino enviroment.
What is the recomendation for Companies that have more than 7500 users in a Domino enviroment and want Cisco UNity UM???
Also note that the maximum supported number of Lotus Domino users supported in a Unity
environment, across all servers, is 7500. Lotus Domino-based environment above this size are not