This is only personal opinion.

If I had two or three books on the desk then I would use either (1) or (3) or (4) or (5).

> (1) I have a couple of books on my desk.  
  (3) I have a number of books on my desk.  
  (4) I have a few books on my desk.  
  (5) I have several books on my desk.

If I had four through nine books on the desk then I would use (3).

> (3) I have a number of books on my desk.

If I had more than nine books on the desk then I would use (6).

> (6) I have many books on my desk.

I would not use (2) with *bunch.*