I am reading docs about AWS aurora postgresql and I am little confused about db instance and replicas. I understand what replica means and it supports 15 replicas in Aurora master cluster. But does one replica map to one db instance? Can I create different numbers of db instance and replicas? If yes, where is each replica located?
But does one replica map to one db instance?
Yes. Your Aurora cluster will have one master db instance and up to 15 replicas. The replicas are db instances just for reading, while master is for writes and reads. Each of these 15 replicates, replicates only one master db instance. You can't make a replica which replicates master of a different cluster.
If yes, where is each replica located?
Not sure what do you mean by that. They are located in subnets which you specified using "subnet group" when you created your cluster.