Simplest way to do that is:

1) dept is a base table for a block
(with select ... from dept ...)
2) in this (dept) block u have to create not database field :cnt for example.
3) create POST-QUERY trigger for block dept,

POST-QUERY trigger:

begin
select count(*) into :dept.cnt from emp where deptno = :dept.deptno;
end;

that is it.