The recent PowerPoint documents are displayed under the Recent menu. The number of recent documents displayed is set to a default value of 20 (PowerPoint 2010) and 25(PowerPoint 2013). This default value can be changed by using Options available as part of PowerPoint Settings. Click on the File menu –> Options link and Navigate to Advanced tab
Scroll down to Display section and using the Show this number of Recent Documents or Presentations, you can change the number of Recent documents displayed under Recent menu. The maximum number of recent documents that can be set s 50.