gistfile1.el

(defun my-filter (condp lst)
  (delq nil
        (mapcar (lambda (x) (and (funcall condp x) x)) lst)))

(defun too-many-buffers-query ()
  (let* ((ls (my-filter (lambda (buf) (buffer-file-name buf)) (buffer-list)))
         (count (length ls)))
    (if (> count 8)
	(y-or-n-p (format "%d file buffers exist: really quit? " count))
      t))
)

(add-hook 'kill-emacs-query-functions 'too-many-buffers-query)