Increase the length of Xcode's "recent project" list?

On a terminal window, for XCode 7 and above type the following command: (from Matteo Gaggiano):

defaults write com.apple.dt.Xcode NSRecentDocumentsLimit X

where X is the max number of projects in list.

For XCode version below 7, the following worked.

defaults write com.apple.Xcode NSRecentDocumentsLimit X

You could change the length of the list with the number of recent items in System Preferences -> General (Appearence on older macOS versions) as well. However that will change it for all apps.

You need to restart your machine for the change to kick in.