"People want me to give hard and fast rules: don't show more than X menu items; don't write more than Y words per page; nothing should be more than Z clicks from the homepage. Sadly, UI design doesn't work that way. Usability questions seldom have a single answer. Rather, they are qualitative issues that specify the direction and nature of inevitable design tradeoffs."
Jakob Nielsen, as part of a post about the additional challenges of usability for mobile devices (my italics).