code: def _find_usage_subnets(self):
          
          # subnets per VPC
          subnet_to_az = {}
          subnets = defaultdict(int)
          for subnet in self.conn.describe_subnets()['Subnets']:
              subnets[subnet['VpcId']] += 1
              subnet_to_az[subnet['SubnetId']] = subnet['AvailabilityZone']
          for vpc_id in subnets:
              self.limits['Subnets per VPC']._add_current_usage(
                  subnets[vpc_id],
                  aws_type='AWS::EC2::VPC',
                  resource_id=vpc_id
              )
          return subnet_to_az
docs: find usage for Subnets; return dict of SubnetId to AZ