不需要子查询：

select dense_rank() over (order by count(*)) as rank,
       country, 
       count(*) as num_user_name
from t
group by country
order by count(*) desc, country;

你可以用这个 dense_rank 分析函数：

with cte as (
    select   country, 
             count(user_name) as num_user_name
    from     tbl
    group by country
), cte2 as (
    select   dense_rank() over (order by num_user_name desc) as ranked,
             num_user_name,
             country
    from     cte
)
select   ranked,
         num_user_name,
         country
from     cte2
where    ranked <= 2
order by 1