with t(reqid, stage) as (
select '15321' , 'start' from dual union all
select '15419' , 'start' from dual union all
select '15537' , null    from dual union all
select '16986' , 'start' from dual union all
select '17051' , 'start' from dual union all
select '17223' , null    from dual union all
select '13856' , 'start' from dual union all
select '13964' , 'start' from dual union all
select '20188' , 'start' from dual union all
select '20347' , null    from dual union all
select '20827' , 'start' from dual union all
select '20988' , 'start' from dual union all
select '20990' , 'start' from dual union all
select '22045' , null    from dual)

select reqid, stage, nvl2(stage, 1 + Sum(decode(Stage, null, 1,0)) over (order by rn), null) num_
from (select reqid, stage, rownum rn from t)