First, Edit > Menus... and make sure that visibility for the Most Recent list is enabled. Also, there will be no list if there has been a rewrite of the InDesign SavedData file due a program update or preferences repalcement, so try saving a file and see if the list reappears.
If it does not, and you still cannot change the number of files to include, it's time to try replacing the prefs. See Replace Your Preferences
Note that you will not see the list (as I mentioned above) until after you start to rebuild it by saving a file, but you should once again be able to specify a number to display.