The question of how many cable boxes is more, for me, a question of DVRs. HOw many different DVRs are needed. Typically, cable DVRs are very limited in storage, especially for HD content. Plus, mom and dad will record many programs that are innappropriate for kids.
So, I try to design around that need. One for Dad, one for Mom, one for the kids, maybe one spare. That way, DVR space for Mom and Dad isn;t wasted with 100 episodes of Sesame street or, my favorite, Thomas the Tank Engine.