from typing import List


def set_intersection(arr_1: List[str], arr_2: List[str]) -> int:
    """
    >>> set_intersection(['1', '2', '3', '4', '5', '6', '7', '8', '9'],
    ... ['10', '1', '2', '3', '11', '21', '55', '6', '8'])
    5
    """
    return len(set(arr_1) & set(arr_2))