i have two different MCS server 7815 and 7835 and we have support 3000 ip phone so how Clustring we do
The MCS 7835 server can support up to 2500 users. However, in order to properly support this number of users you are going to need at least 3 MCS 7835 Servers.
One server to be the publisher, 1 server to be the subscriber & 1 server as the backup subscriber. You can use the MCS7815 as a dedicated TFTP server or as a dedicated Music on Hold Server.
When you install the Call Manager software it will ask you is this is going to be the "first" node in the cluster or if you are going to add a "node" (i.e. a subscriber) to the cluster.