SQL> create table t1 ( x date )
  2  partition by range ( x )
  3  interval ( numtodsinterval(7,'DAY'))
  4  (
  5    partition p1 values less than ( date '2017-04-01' )
  6  );

Table created.

SQL>
SQL> insert into t1
  2  select sysdate-10+rownum
  3  from dual
  4  connect by level <= 20 ;

20 rows created.

SQL>
SQL> exec dbms_stats.gather_table_stats('','T1');

PL/SQL procedure successfully completed.

SQL>
SQL> select partition_name, num_rows
  2  from user_tab_partitions
  3  where table_name = 'T1';

PARTITION_NAME                   NUM_ROWS
------------------------------ ----------
P1                                      4
SYS_P725                                7
SYS_P726                                7
SYS_P727                                2

4 rows selected.