SQL>  with
  2        data as (
  3          select number_from, number_to,
  4                 lag(number_to) over ( order by number_from) prev_number_to
  5          from tt
  6        )
  7      select number_from, number_to, prev_number_to,
  8             decode(number_from-prev_number_to,2,to_char(prev_number_to+1)||' is missing',
                       'Nothing is missing') extra
  9      from data
 10  /

NUMBER_FROM  NUMBER_TO PREV_NUMBER_TO EXTRA
----------- ---------- -------------- ---------------------------------------------------
          1          3                Nothing is missing
          5          9              3 4 is missing
         10         15              9 Nothing is missing

SQL>

SQL> with
  2    limits as (
  3      select min(number_from) minnum, max(number_to) maxnum from t
  4    ),
  5    numbers as (
  6      select minnum+level-1 num from limits connect by level <= maxnum-minnum+1
  7    )
  8  select num from numbers
  9  minus
 10  select num
 11  from t, numbers
 12  where num between number_from and number_to
 13  order by 1
 14  /
       NUM
----------
         4

select b.ide from a,(select rownum ide from dual connect by level<=100) b
where a.ide(+)=b.ide
and a.ide is null

SQL> desc ttt
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 NUMBER_FROM                                                    VARCHAR2(10)
 NUMBER_TO                                                      VARCHAR2(10)

SQL> select * from ttt
  2  ;

NUMBER_FRO NUMBER_TO
---------- ----------
1          3
5          9
10         15
16         19
23         24

SQL>  with
  2        limits as (
  3          select min(to_number(number_from)) minnum, max(to_number(number_to)) maxnum from ttt
  4        ),
  5        numbers as (
  6          select minnum+level-1 num from limits connect by level <= maxnum-minnum+1
  7        )
  8   select num from (
  9          select minnum+level-1 num from limits connect by level <= maxnum-minnum+1
 10        )
 11      minus
 12      select num
 13      from ttt, numbers
 14      where num between to_number(number_from) and to_number(number_to)
 15      order by 1
 16   /

       NUM
----------
         4
        20
        21
        22

SQL>