Collector clusters are groups of one or more collectors that are responsible for monitoring their assigned monitored hosts. Monitored hosts are always assigned to a collector cluster, rather than individual collectors. All collectors in a cluster are jointly responsible for monitoring their assigned hosts.
Clustering collectors together allows monitoring more hosts by sharing the monitoring load. Clustering also provides monitoring resilience, reducing the chances of losing monitoring visibility if there is a problem with individual collectors in a cluster. If a collector goes down, the remaining collectors provide continuity of monitoring as they pick up the slack until the problem collector is restored.
Collector clusters should always have an odd number of collectors, this is important as it ensures you have an effective and supported collector cluster configuration.