Chapter Manager is limiting to only show 500 members at a time
The chapter manager part uses a directory for the member search. If you edit a chapter manager part, you will see that the tab is noted as the Member Search tab and uses the Directory Search Editor.
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 members feature of the chapter manager part was intended to be a search tool, rather than display the whole list of members.
The members list can still be searched throughout the entire list. For example: If the chapter manager stops displaying members with last name that begins with 'F,' we can still search members who have a last name that begins with the letter 'M.' The results will still pull from the whole list of members.