Count column values which appear in more than one table row with PSQL

I have table like this:

I need get numbers, wich is repeated in different citys and also get these citys quantity for each number.

That is from this table, I need result:

number | repeated citys quantity
 ----------
222    | 2

because number 222 is repeated in 2 different city.

I have this solution:

1) greate function which returns unique values from array, for example array_unique()

2) and then:

SELECT number, array_length(uniq_city_list, 1) FROM (
    SELECT number, array_unique(array_agg(city)) AS uniq_city_list FROM mytable
    GROUP BY number
)
AS t
WHERE array_length(uniq_city_list, 1) > 1

But, may be there is better solution for doing this? I think this is not optimal query...

select number, count(*)
from (
    select number, city
    from t
    group by number, city
) s
group by number
having count(*) > 1
order by number