count-comments-by-user.sql

SELECT u.ID, u.user_email, count(distinct ifnull(c.comment_ID, 0)) as count,
(
	SELECT GROUP_CONCAT( m.meta_value )
	FROM wp_usermeta m
	WHERE m.meta_key = 'member_level' AND m.user_id = u.ID AND m.meta_value != 'Non-member'
) as member_level
FROM wp_users u
JOIN wp_comments c ON u.ID = c.user_id
WHERE comment_approved = 1 AND comment_type IN ("comment", "") AND c.user_id <> 0
GROUP BY u.ID
ORDER BY count DESC

Use this to also check if they've already got a setting to load all the comments. This puts them to the bottom if they do have that field, no matter how many comments they've got:

SELECT u.ID, u.user_email, count(distinct ifnull(c.comment_ID, 0)) as count,
(
	SELECT m.meta_value
	FROM wp_usermeta m
	WHERE m.meta_key = 'always_load_comments' AND m.user_id = u.ID
) as comment_load_status
FROM wp_users u
JOIN wp_comments c ON u.ID = c.user_id
JOIN wp_usermeta m ON u.ID = m.user_id
WHERE comment_approved = 1 AND comment_type IN ("comment", "") AND c.user_id <> 0
GROUP BY u.ID
ORDER BY comment_load_status ASC, count DESC

Note: for this purpose, we don't care about the member level so it's not included.