def get_num_label_vols(md):
    """
    Get the number of volumes used for labelling - e.g. 2 for tag-control pair data
    """
    iaf = md.get("iaf", "tc")
    if iaf in ("tc", "ct"):
        return 2
    elif iaf == "diff":
        return 1
    elif iaf == "mp":
        return md.get("nphases", 8)
    elif iaf == "ve":
        return md.get("nenc", 8)