SELECT ad_id, count(distinct(user_id))
FROM reachcount
GROUP BY ad_id;

queryset = ReachCount.objects.values('ad_id').annotate(Count('user_id',distinct=True))

SELECT `test_reachcount`.`ad_id`, COUNT(DISTINCT `test_reachcount`.`user_id`) AS `user_id__count`
FROM `test_reachcount`
GROUP BY `test_reachcount`.`ad_id`, `test_reachcount`.`user_id`
ORDER BY `test_reachcount`.`ad_id` ASC, `test_reachcount`.`user_id` ASC