Amazon ElastiCache for Redis
ElastiCache for Redis User Guide (API Version 2015-02-02)

Changing the Number of Replicas

You can dynamically increase or decrease the number of read replicas in your Redis replication group using the AWS Management Console, the AWS CLI, or the ElastiCache API. If your replication group is a Redis (cluster mode enabled) replication group, you can choose which shards (node groups) to increase or decrease the number of replicas in.

To dynamically change the number of replicas in your Redis replication group, choose the operation from the following table that fits your situation.