If you are running several hundred DB instances, it is always best practice to schedule an automation around RDS service limits. Get-RDSAccountAttribute lets you list all the attributes and limits for your RDS Service in your account. This will list all the current quotas and the maximum usage allowed in your account for the RDS service.
PS C:\> Get-RDSAccountAttribute
AllocatedStorage Used limit does not account for the standby site. The utilization displayed is for the primary site. You can still request the AWS raises the limit if needed. But monitoring these limits is essential for your day-to-day operation.