select pk_seq.nextval
  from all_objects
 where rownum <= :num_of_values;

declare
  type na is table of number index by binary_integer;
  v_na  na;
begin
  select foo.nextval bulk collect
    into v_na
    from all_objects
   where rownum <= 5;
  for i in 1..v_na.count loop
    dbms_output.put_line( v_na(i) );
  end loop;
end;