How to sort by count with postgresql?

You’ve aliased the table and column as the same thing, so don’t do that. It’s not invalid, just tough to follow.

Anyway, include all columns that you’re selecting that aren’t aggregates in your group by:

select 
    count(w.id) as mycount,
    w.company_id,
    c.company_name,
    c.city
from 
    companies c 
    left join workers w on 
        c.id=w.company_id 
group by 
    w.company_id,
    c.company_name,
    c.city
order by mycount desc;