You can now use Amazon Neptune Auto Scaling to automatically add or remove Read Replicas in response to changes in performance metrics you specify. Neptune Read Replicas share the same underlying volume as the primary instance and are well suited for read scaling. With Neptune Auto Scaling, you can specify a desired value for CloudWatch (CW) metrics for your Replicas such as average CPU utilization. Neptune Auto Scaling adjusts the number of Read Replicas to keep the CW metric closest to the value you specify.
Source:: Amazon AWS