Acrobat reader displays most recently used documents in a section under the File menu.
The default number of maximum documents displayed under recently accessed document section is 5. This can be changed by using the settings available under Preferences menu.
Click on the Edit menu –> Preferences (Ctrl + K)
Navigate to Startup Categories, under Opening Documents section using Maximum documents in most-recently used list you can change the default value specified for the maximum number documents that can be displayed under most recently used list.