В Drupal 8 таблица comment_field_data содержит cid (Comment Id) и uid (ID пользователя автора).
SELECT count(cid), uid FROM comment_field_data GROUP BY uid;
Включение заголовков столбцов и добавление имени пользователя:
SELECT COUNT(cid) AS 'Total # of Comments', comment_field_data.uid AS 'User Id', users_field_data.name as 'Username'
FROM comment_field_data
LEFT JOIN users_field_data ON comment_field_data.uid = users_field_data.uid
GROUP BY comment_field_data.uid;