SQL> CREATE SEQUENCE TEST_OBJ_SEQ
INCREMENT BY  1 
START WITH    1 
MINVALUE      1 
NOCYCLE
ORDER
NOCACHE
/
Sequence created.

SQL> create or replace type tb_numbers as table of number;
/
Type created.

SQL> create or replace
function gen_indexes( p_num number ) return tb_numbers is
 tb tb_numbers := tb_numbers();
 j  pls_integer;
begin
 for j in  1 ..p_num loop
   tb.extend;
   select test_obj_seq.nextval into tb(j) from dual;
 end loop;
 return tb;
end;
/
Function created.

SQL> select * from table(cast(gen_indexes( 10 ) as tb_numbers));
COLUMN_VALUE
 ------------
 
	    2 
	    3 
	    4 
	    5 
	    6 
	    7 
	    8 
	    9 
	   10 
	   11 

 10  rows selected.
SQL> select * from table(cast(gen_indexes( 5 ) as tb_numbers));
COLUMN_VALUE
 ------------
 
	   12 
	   13 
	   14 
	   15 
	   16