I don't know that there's any hard and fast rule for the number of hard answers/clues that make a puzzle difficult. For one thing, a hard answer can be really easy with the right clue, and a really easy answer can be devilish with a challenging clue. Plus, there are aspects of the puzzle's shape that make things harder or easier that are independent of the difficulty of each individual answer or word. For example, puzzles with very few black squares are generally harder because the fill tends to be "denser" (i.e. larger chunks of fill). Puzzles with more "Theme squares" tend to be harder because theme answers tend to be harder to solve and the more theme squares there are, the more often the constructor will have to resort to less-than-stellar fill. Other aspects of a puzzle that make it harder is the average word length (longer words are generally harder to solve than shorter words), words with more proper nouns (these often fall in the "you know it or you don't" category), and puzzles with more open squares (squares that don't touch black squares in any direction). To rank puzzles by difficulty would require quantifying these aspects of a puzzle's geometry in addition to taking into account a puzzle's clues/answers. Or, at least, that's my opinion!
Oh, and some theme types are generally harder than other theme types. For example "quote" or "pun" puzzles tend to be pretty challenging, as are rebus puzzles.
