create table asher( num varchar(15));
insert into asher values('26SI6D0001');
insert into asher values('26SI6D0002');
insert into asher values('26SI6D0003');
insert into asher values('26SI6D0005');
insert into asher values('26SI6D0006');
insert into asher values('26SI6D0007');
insert into asher values('26SI6D0008');
insert into asher values('26SI6D0010');

with tab as
(select to_number(replace(num,'26SI6D')) su from asher)
select fs from (select '26SI6D'||to_char(lpad(rownum,4,0)) fs from dba_tables  where rownum<=(select max(su) from tab))
where fs not in (select num from asher)

SQL> ed
Wrote file afiedt.buf

  1  with tab as
  2  (select to_number(replace(num,'26SI6D')) su from asher)
  3  select fs from (select '26SI6D'||to_char(lpad(rownum,4,0)) fs from dba_tabl
es  where rownum<=(select max(su) from tab))
  4* where fs not in (select num from asher)
SQL> /

FS
----------
26SI6D0004
26SI6D0009

select '26SI6D'||to_char(lpad(rownum,4,0)) fs from <...>

select '26SI6D'||to_char(rownum,'fm0999') fs from <...>

select * from (
	select '26SI6D'||to_char(mn + level -1, 'fm0999') val
	from (
		select min(replace(invoice_id, '26SI6D')) mn
			, (max(replace(invoice_id, '26SI6D')) - min(replace(invoice_id, '26SI6D'))) btwn
		from inv)
	connect by level <= btwn + 1)
where val not in (select invoice_id from inv);

SELECT '26SI' -- prefix
     ||substr(to_char(input_date, 'YYYY'), 4, 1) -- year
     ||chr(to_number(to_char(input_date, 'MM'))+64) -- month
     ||to_char(mn + level -1, 'fm099') val

set pages 0
select'26SI06'||rownum from dual
where rownum < 100010
connect by 0=0
/

select'26SI06'||rownum from dual
connect by 0=0
and rownum < 100010
/

SELECT '26SI06'||to_char((rownum+1-1),'fm0999' )
FROM dual
CONNECT BY rownum < 100010;