def METHOD_NAME(self, subtask: Subtask):
    """
    Get the number of shuffle blocks that a mapper operand outputs,
    which is also the number of the reducers when tiling shuffle operands.
    Note that this might be greater than actual number of the reducers in the subtask graph,
    because some reducers may not be added to chunk graph.
    Parameters
    ----------
    subtask : Subtask
        A mapper or reducer subtask.
    Returns
    -------
    n_reducers : int
        The number of shuffle blocks that a mapper operand outputs.
    """
    index = self._mapper_indices.get(subtask) or self._reducer_indices.get(subtask)
    if index is None:
        raise ValueError(f"The {subtask} should be a mapper or a reducer.")
    else:
        shuffle_index, _ = index
        return self._mapper_output_refs[shuffle_index].shape[1]