This is only personal opinion.

If I had one or 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.*