declare @t table (id int identity, num int)

insert into @t (num)
select 11 union all
select 12 union all
select 33 union all
select 44 union all
select 15 union all
select 100 union all
select 100 union all
select 2 union all
select 3 union all
select 4 union all
select 100

select *, 
row_number() over(partition by (select count(*) from @t t2 where t2.num=100 and t2.id<=t1.id) order by t1.id) as xz
from @t t1

id          num         xz
----------- ----------- --------------------
1           11          1
2           12          2
3           33          3
4           44          4
5           15          5
6           100         1
7           100         1
8           2           2
9           3           3
10          4           4
11          100         1

(11 row(s) affected)