My rule of thumb is keep the front pages down to under 5-7 if possible, depending on the type of the site.
Interior pages, depending on what they need might have more, but I do what I can to keep it under 20.
However, at the same time, depending on what you are trying to do AND what types of caching you are doing with that information 20 may not be bad if 15 of them are heavily cached...
