Like you, I had set iCal's week view to 14 days using the Debug menu in iCal under Lion before upgrading to Mountain Lion. However, at least for me, changing the value via Calendar>Preferences... did not permanently change the setting. When I restarted Calendar, it always reverted to the previous value (in my case, 14 days) if the value was not one of those available from the menu (i.e., 5 or 7).
There is another way to set the number of days shown in week view, and that's to use the following shell command:
defaults write com.apple.iCal CalUIDebugDefaultDaysInWeekView 7
This works where Calendar>Preferences... does not, for reasons I don't understand.
The "7" in the command above can be another number, including ones not previously available from Lion iCal's Debug menu, e.g., 10. However, I find that Mountain Lion's Calendar has a peculiar bug when the value is not 5 or 7: in week view, if you have moved the viewed range of dates such that today is not visible, then using View>Go To Today (or Command+T) does not move the days in the grid to show today. The title changes to the correct dates, but the days in the grid don't change to the correct range of dates.
(Hat tip to this posting for pointing out that the numerical value could be anything, not just a value shown in iCal's Debug menu.)
Best Answer
Sadly I think you are out of luck when it comes to this. Like most Apps that comes with OS X there isn't much space for customizing the UI.