Below is an extract from the latest CCM book
"A CUCM cluster can have up to 20 servers in it. Only one publisher server is allowed in the cluster. The publisher houses the read/write copy of the database. Up to eight subscriber servers can be in the cluster, with the restriction that only four of the subscriber servers can perform active call processing. If more than four subscriber servers are used in a cluster, the additional servers are dedicated standby servers in case the active subscriber server is not available. The other 11 servers in the cluster can be responsible for various services, including TFTP and media resources (conferencing, music on hold, transcoding)"
I always thought you can have 8 x servers running CCM Service in a single Cluster, has this changed with 5.x, 6.x ? or is it just referencing a 1:1 redundancy model i.e 4 active servers with a redundant server for each active one