in the RDS example, there are two standby Maria DB, is it supported to have multiple standby RDS instances to the other AWS RDS
In the example you’re talking about, those other MariaDB RDS databases are Read Replicas which are different than "standby" instances. You can promote a read replica to the master though.
For a multi-AZ RDS deployment, you do have standby’s that are located in other AZ’s and will automatically assume the Master role if the master fails. That’s the distinction between read replicas and multi-AZ.
To the best of my knowledge, and based on the documentation, multiple standby (Multi-AZ) instances ARE NOT support for non-Aurora databases. For Aurora databases, we know that they replicate it across 3 AZ. But if it is not Aurora, you can only have 1 standby database.
See the documentation here: