SQL - How To Order Using Count From Another Table

Use subqueries.

select * from (
    select post_from_blogger_id, count(1) N from Posts
    group by post_from_blogger_id) t
order by N desc

Try this:

SELECT B.blogger_id,
       B.blogger_name,
       IFNULL(COUNT(P.post_from_blogger_id ),0) AS NumPosts 
From Blogger AS B
LEFT JOIN Posts AS P ON P.post_from_blogger_id = B.blogger_id
GROUP BY B.blogger_id, B.blogger_name
ORDER BY COUNT(P.post_from_blogger_id ) DESC

This joins the 2 tables, and counts the number of entries in the Posts table. If there are none, then the count is 0 (IFNULL).

 SELECT bloggers.*, COUNT(post_id) AS post_count
    FROM bloggers LEFT JOIN blogger_posts 
    ON bloggers.blogger_id = blogger_posts.blogger_id
    GROUP BY bloggers.blogger_id
    ORDER BY post_count

(Note: MySQL has special syntax that lets you GROUP BY without aggregating all values, it's intended for exactly this situation).