If you have Data Explorer plug-in installed, I think this will work:
-- [params]
-- date :date_from =
-- date :date_to =
-- int :topic_id =
SELECT COUNT(p.id) as posts_count
FROM posts p
WHERE p.created_at::date BETWEEN :date_from::date AND :date_to::date
AND p.topic_id = :topic_id
AND p.deleted_at IS NULL
The query counts the number of posts in the specified topic that were created within the date range and have not been deleted, so you can use any period of time. Date format is YYYY-MM-DD and topic id can be seen in the topic’s URL.