Display department name,the number of employees and their names
You need to fetch the number of employees in a sub-query and then use this:

select 
  q.total as "num of employees", 
  d.department_name, 
  e.employee_name
from 
  departments d join 
  employees e on d.dept_id=e.dept_id join    
  (
    select 
      count(dept_id) as total, 
      dept_id
    from     
      employees
    group by department_id
    ) q on d.dept_id=q.dept_id
group by d.department_name, e.employee_name