ORA-01008: not all variables bound

declare
  cnt number;
begin
   execute immediate
      'select
          count(*)
       from
          dual
       where
          1 = :a and
          2 = :b'
   into cnt
   using 1;
end;
/

SQL> set define off
SQL> var res varchar2
SQL> exec execute immediate 'select dummy from dual where 1 = &num' into :res;