@classmethod
def get_number_of_runs(cls, subject_id):
    """Return the number of runs for the given subject.

        Parameters
        ----------
        subject_id : str
            The subject id

        Returns
        -------
        n_runs : int
            The number of runs for the subject.
        """
    n_runs = 1
    while cls.get_fif_filename(subject_id, n_runs) is not None:
        n_runs += 1
    return n_runs - 1
