When searching in a Directory Part, only 500 results show at a time. You know there may be more results in the directory, but they are not displaying.
The Directory search can return up to 500 results that match the criteria entered by a site user. This limit was set up for performance reasons. The user can add additional criteria in order to narrow down the results.
For example, if they search the directory for last name "S", and 500 results show, but their desired result in not returned, they can search for "Sm" to narrow the results.