Updates num_update.

        Parameters
        ----------
        index : int or list of int
            The index to be updated.

def _update_count(self, index):
        """Updates num_update.

        Parameters
        ----------
        index : int or list of int
            The index to be updated.
        """
        if not isinstance(index, (list, tuple)):
            index = [index]
        for idx in index:
            if idx not in self._index_update_count:
                self._index_update_count[idx] = self.begin_num_update
            self._index_update_count[idx] += 1
            self.num_update = max(self._index_update_count[idx], self.num_update)