def _find_usage_instances(self):
        """calculate On-Demand instance usage for all types and update Limits"""
        # update our limits with usage
        inst_usage = self._instance_usage()
        res_usage = self._get_reserved_instance_count()
        logger.debug('Reserved instance count: %s', res_usage)
        total_ris = 0
        running_ris = 0
        # subtract reservations from instance usage
        ondemand_usage = defaultdict(int)
        for az in inst_usage:
            if az not in res_usage:
                for i_type, count in inst_usage[az].items():
                    ondemand_usage[i_type] += count
                continue
            # else we have reservations for this AZ
            for i_type, count in inst_usage[az].items():
                if i_type not in res_usage[az]:
                    # no reservations for this type
                    ondemand_usage[i_type] += count
                    continue
                od = count - res_usage[az][i_type]
                total_ris += res_usage[az][i_type]
                if count < res_usage[az][i_type]:
                    running_ris += count
                else:
                    running_ris += res_usage[az][i_type]
                if od < 0:
                    # we have unused reservations
                    continue
                ondemand_usage[i_type] += od
        logger.debug(
            'Found %d total RIs and %d running/used RIs',
            total_ris, running_ris
        )
        total_instances = 0
        for i_type, usage in ondemand_usage.items():
            key = 'Running On-Demand {t} instances'.format(
                t=i_type)
            self.limits[key]._add_current_usage(
                usage,
                aws_type='AWS::EC2::Instance',
            )
            total_instances += usage
        # limit for ALL On-Demand EC2 instances
        key = 'Running On-Demand EC2 instances'
        self.limits[key]._add_current_usage(
            total_instances,
            aws_type='AWS::EC2::Instance'
        )
calculate On-Demand instance usage for all types and update Limits