Count values to show up with zeros instead of null
You need to join in the opposite way: managers LEFT JOIN employees

create view scnt as 
    select E2.id as super, 
           count(E1.id) as cnt
    from Employee E2                    -- managers
      left outer join Employee E1       -- employees
      on E1.super = E2.id
    group by E2.id
  ;

select * from scnt ;


dbfiddle