Natalie reminds him that the cases of Douglas Thurman and James Novak are separate from one another, meaning that Monk has in fact solved 101, meaning he can't retire until he has solved another 99 cases (200) overall.
Natalie reminds him that the cases of Douglas Thurman and James Novak are two completely different cases although they are connected. Hence, Monk has in fact solved 101 cases, meaning that if he wants to retire, he must solve another 99 cases (200) overall.