How many files will Excel show in the Recently Used File List? Well, it depends on the version of Excel you're using and vary between 9 and ninety-nine files.
Here is a list of Excel versions, menu navigation to the recent file list, and the max files allowed.
Excel 2010, 2007 File>Options>Advanced tab>under the Display heading: Show this number of Recent Documents: 50 max

Excel 2003 Tools>Options>General tab Recently used file list: 9 max

Excel for Mac 2008, 2011 Excel>Preferences>General Show this number of recent documents: 99 max

A Related Feature in Excel 2010
At the bottom of the Recent Workbooks list, Excel 2010 has an option you can check to: Quickly access this number of Recent Workbooks: x
Checking this box adds recent files to the left pane so you can always see the file names. They're not really the most recent files because they include any files you have pinned to the top of the list, even if they haven't been opened in a while.
(I covered this feature in a microblog article on Tumblr and in a recent post with video here on this blog.)