This is hard to do automatically. What you can do, though, is have your application keep track of the count of items as they are added; when you hit your maximum, instantiate a cascade button that says "More" (instead of a push button), create the associated pulldown menu pane, and then continue to populate that pane. Recurse as necessary!

Note, however, that this solves only the very narrow UI problem. The real problem is that you have too many items in the menu. It may be possible to differentiate them so that they are separated by subject, as items on the File menu differ from those on the View menu. It may also be more appropriate to have several buttons simultaneously visible on the screen, perhaps in a separate window. It`s hard for people to make choices among too many items unless the groupings are smaller.