SQL> with t(id, parent_id, name)
  2       as (select 1,null,'Russia'from dual
  3           union all select 2,1,'Moscow'from dual
  4           union all select 3,2,'Lenina'from dual
  5           union all select 4,2,'Street2'from dual),
  6       h
  7       as (select id,
  8                  parent_id,
  9                  name,
 10                  level l,
 11                  rownum rn
 12             from t
 13           start with parent_id is null
 14           connect by parent_id = prior id)
 15  select h0.*, count(*) over (order by s range between current row and x - 1e-38 following) cnt
 16    from (select h.*,
 17                 power(10, 1 - l) x,
 18                 sum(power(10, 1 - l)) over (order by rn) s
 19            from h) h0;

        ID  PARENT_ID NAME             L         RN          X          S        CNT
---------- ---------- ------- ---------- ---------- ---------- ---------- ----------
         1            Russia           1          1          1          1          4
         2          1 Moscow           2          2         .1        1.1          3
         3          2 Lenina           3          3        .01       1.11          1
         4          2 Street2          3          4        .01       1.12          1

SQL>