We use the "Table of contents" web part on SharePoint landing pages to give access to users to easily navigate to the sites/pages. But, there is a default limitation on the display items in left navigation or table of web part contents in sharepoint. By default it shows only 50. If you have more than 50 items [either sub sites or pages] in a site then it will not show up. To increase the limit or to remove limit, please do below changes.