If you want to get tags based on category, and order by tag count value, here is the query:
Assuming that the category ID is 123456
SELECT wp_term_taxonomy.term_id as term_id, COUNT(*) AS count FROM wp_term_taxonomy INNER JOIN wp_term_relationships ON wp_term_relationships.term_taxonomy_id = wp_term_taxonomy.term_id WHERE wp_term_taxonomy.taxonomy = 'post_tag' AND wp_term_relationships.object_id IN ( SELECT `object_id` FROM `wp_term_relationships` WHERE `term_taxonomy_id` = 123456 ) GROUP BY term_id ORDER BY `wp_term_taxonomy`.`term_id` DESC
Will return (sample):
Notice that the count here is not default WordPress term count, but count of returning rows of same values, for posts under caterogy of 123456.
Warning: This is the raw query phpMyadmin, you need to do all the security prepare stuffs.
There is another version using the built-in WordPress functions: