Easiest way is probably a Query field using CB Query Field then just query for a count of those comments. Example as follows.
Code:
SELECT COUNT(*)
FROM `#__comprofiler_plugin_activity_comments` AS c
WHERE c.`user_id` = USER_ID_HERE
AND c.`asset` IN ( SELECT CONCAT( 'activity.', a.`id` ) FROM `#__comprofiler_plugin_activity` AS a WHERE a.`asset` = 'ASSET_HERE' )
Ideally this would just have an inner join of the activity to filter things down, but without the object and target columns (not implemented yet) the query is a little odd, but it works if you want to use that method instead.
Code:
SELECT COUNT(*)
FROM `#__comprofiler_plugin_activity_comments` AS c
INNER JOIN `#__comprofiler_plugin_activity` AS a
ON a.`id` = SUBSTRING( c.`asset`, 10 )
AND a.`asset` = 'ASSET_HERE'
WHERE c.`user_id` = USER_ID_HERE
AND c.`asset` LIKE 'activity.%'