As window functions are evaluated after grouping, it is possible to use aggregating functions as arguments for window functions, but not the other way around.

A code like the following is correct: sum(count(*)) OVER()

This will also work: sum(a) OVER(ORDER BY count(*))

However, the code, sum(count(*) OVER()), is wrong.

For example, to calculate the rank of the seller accounts by the number of advertisements they give, the following query can be used:

car_portal=> SELECT seller_account_id, dense_rank() OVER(ORDER BY count(*) DESC)
 FROM car_portal_app.advertisement
 GROUP BY seller_account_id;
 seller_account_id | dense_rank
-------------------+------------
                26 |          1
               128 |          2
                28 |          2
               126 |          2
...