altering the default number of songs in upnext (itunes11, osx)? Currently I only see about 12 songs in upnext, not so great when I want to create a 3 hour playlist, but I can't find a setting anywhere to increase this.

Does such a thing exist?
You can make a normal or intelligent playlist and play it. Do not use shuffle and you can add songs during playback.

Edit: If you just want to show more songs in upnext - it is not possible. But 12 Songs i s a plenty of view in the future....