Sample k times out of alphabet, how many different?

def uniqueof20(k, rep=10000):
    """Sample k times out of alphabet, how many different?"""
    alphabet = 'ACDEFGHIKLMNPQRSTVWY'
    reps = [len(set(random.choice(alphabet)
                    for i in range(k)))
            for j in range(rep)]
    return sum(reps) / len(reps)