(require 'dash)

(defmacro with-file (file &rest body)
  "Open FILE, execute BODY close FILE if it was not already open."
  `(let ((old-buffer (current-buffer))
         (kill-buffer-p (not (get-file-buffer ,file))))
     (unwind-protect
         (progn
           (find-file ,file)
           ,@body)
       (progn
         (when kill-buffer-p (kill-buffer))
         (switch-to-buffer old-buffer)))))

(--> org-agenda-files
     (--map (with-file it (org-get-buffer-tags)) it)
     -flatten
     -distinct
     length)