So there I am, looking in the Users OU within Active Directory Users and Computers, and I notice that not all of the users are listed. The list only went to names that start with the letter N. So why wasn’t I seeing everything?
It turns out that Active Directory Users and Computers will only display 2000 items per folder. A setting within the Filter Options controls this. You can access is by selecting View > Filter Options… Once you are there, just set the Maximum number of items displayed per folder setting to a higher number. I didn’t want anything filtered, so I set it to 20,000.