Illustrator - changing number of recent files

It should work, so I would recommend resetting your Illustrator preferences.

To do so on a PC:

You can try the quick way of resetting on a PC which is to hold down Ctrl + Alt + Shift when launching Illustrator and respond affirmatively when asked if you want to reset. There have been some recent reports that the window asking if you want to reset is not popping up but that the prefs are being reset anyway. If this works then great but if it doesn’t you may have to manually delete them.

On Windows 7 and above the preference files are hidden. To find them go to the Control Panel and open Folder Options and then click the View tab. Then select “Show hidden files and folders” or “Show hidden files, folders or drive options” in Advanced Settings (you may be able to skip the previous steps with Windows 10). Then delete (or rename) the folder at the end of this path: C:\Users\[User Name]\AppData\Roaming\Adobe\Adobe Illustrator [version number]\ Settings\<Language> for any versions that you have. Make sure that Illustrator is closed when you do this. When you relaunch the program it will create  new preference files and the program will be at its default settings.

After you’ve reset up the program (make sure that no document window is open) to your liking, it is a really good idea to create copies of your personalized “mint” preference files (make sure that you quit the program before copying them—that finalizes your customization) and use them in the future to replace any corrupt versions you may need to delete.

After you reset, hopefully, you can go into your preferences and set up the recent files option to your liking.