code: def _find_usage_instances(self):
          
          paginator = self.conn.get_paginator('describe_db_instances')
          for page in paginator.paginate():
              for instance in page['DBInstances']:
                  self.limits['Read replicas per master']._add_current_usage(
                      len(instance['ReadReplicaDBInstanceIdentifiers']),
                      aws_type='AWS::RDS::DBInstance',
                      resource_id=instance['DBInstanceIdentifier']
                  )
docs: find usage for DB Instances and related limits