Key systems installed in offices usually have only one or two phones ringing for the listed directory number calls, and other phones that ring only for a personal line. So cascading is not usually a problem.
I think the ring generator is too small to ring more than 4 ports at once (up to two phones per port).
[EDIT: MAX 6 STANDARD PHONES PER RINGING GROUP]
You can always put phones within hearing range on every 4th station port (1st, 5th, 9th, 13th or 2nd, 6th, 10th, 14th, etc.) Or put similar sounding phones together: for example, 8A ringers (candlesticks) on 1,5,9,13; 78A ringers (202s with 684A subsets) on 2,6,10,14; B1AL ringers (302s) on 3,7,11,15; C4A ringers (500s) on 4, 8, 12,16.
Does the cascading happen only on single line ("Industry Standard") phones or also on the proprietary multi-line Panasonic phones?