What is the minimum number or databases that can exist within a cluster when deploying...

 The minimum number of databases that can exist within a cluster when deploying a resilient and scalable Cisco Meeting Server deployment is 3. This is because a database cluster of 3 nodes can tolerate the failure of one node and still maintain a quorum, which is the majority of nodes that are online and can agree on the state of the data. A database cluster of 2 nodes cannot tolerate any failure, as it would lose the quorum and become unavailable. A database cluster of more than 3 nodes can also work, but it would consume more resources and increase the complexity of the deployment1.

References := 1: Frequently Asked Questions - Why do I need a database cluster with at least 3 nodes rather than 2 for my Meeting Server deployment?