def _get_number_of_children(base, tag):
    """Count all child elements with a given tag.

    Args:
        base: The base node element. (lxml node)
        tag: Child elements group tag used to select the elements. (string)

    Returns:
        A int number of the found child elements with the id.
    """
    counter = 0
    for node in base:
        if node.tag.replace('{http://www.rdml.org}', '') == tag:
            counter += 1
    return counter
