select count(r.reg_no) as Total_Registration -- results in 10*2 = 20
     , count(r.reg_no) - count(c.cancellation_id) as actual_Attendance -- results in 20-20 => 0
  from registration r
     , cancellation c;

SQL> select count(*) from emp;

  COUNT(*)
----------
        14

SQL> select count(*) from dept;

  COUNT(*)
----------
         8

SQL> select e.cnt cnt_emp
  2       , d.cnt cnt_dept
  3       , e.cnt-d.cnt test
  4    from ( select count(*) cnt
  5             from emp
  6         ) e
  7       , ( select count(*) cnt
  8             from dept
  9         ) d
 10  /

   CNT_EMP   CNT_DEPT       TEST
---------- ---------- ----------
        14          8          6

SQL>