How many replicas do you need?
Having 3 replicas per datacenter is a good starting point for
relatively small clusters. As the number of nodes in a datacenter becomes larger,
a higher replication factor may become a better choice.
The number of replicas can affect consistency, availability, latency and throughput. Increasing a replication factor improves availability as it becomes possible to tolerate more replica failures. Also, a larger set of replicas can serve more concurrent requests and result in better response times.