CODE:
drop trigger Validation_volunteer;
create or replace trigger Validation_volunteer
before insert on v_project for each row
declare
  v_count number;
begin
  select count(v_id)
  into   v_count
  from   v_project
  where  pro_id = :new.pro_id;


  if (v_count > 50) then
    raise_application_error(-20000, 'ERROR!! Validation is for inserting the number of volunteers , which will be 50');
  end if;
end;