def collatz_sequence_length(num: int) -> int:
    """ Returns the length of the Collatz sequence starting at a given number """
    if num == 1:
        return 1

    if num % 2 == 0:
        return collatz_sequence_length(num // 2) + 1
    else:
        return collatz_sequence_length(3 * num + 1) + 1