def longest_chain(lst):
    if not lst:
        return None

    curr = lst[0]
    count = 1
    prev = count
    max_num = curr

    for number in lst[1:]:
        if number != curr:
            if count > prev:
                max_num, prev = curr, count
            curr = number
            count = 1
        else:
            count += 1

    if count > prev:
        max_num, prev = curr, count

    return max_num, prev

print(longest_chain([6, 1, 6, 4, 4, 4, 6, 6]))
print(longest_chain([1, 1, 1, 2, 2, 2, 2, 1, 1, 1]))
print(longest_chain([1, 2, 2, 2, 1, 1, 1, 1]))
print(longest_chain([]))