引数の数を調べる

arity-example.el

(arity (symbol-function 'identity)) ;=> (1 . 1)
(arity (symbol-function '+)) ;=>  (0 . many)
(arity (symbol-function 'substring)) ;=> (2 . 3)

(defun foo (a b c &optional d e f))
(arity (symbol-function 'foo)) ;=> (3 . 6)

(defun bar (a b c &rest rest))
(arity (symbol-function 'bar)) ;=> (3 . many)

arity.el

(defun arity (func)
  "Return minimum and maximum number of args"
  (if (subrp func)
      (subr-arity func)
    (let ((min 0) (max 0) optional)
      (catch 'many
        (mapcar
           (lambda (arg)
               (cond ((eq arg '&optional)
                      (setq optional t))
                     ((eq arg '&rest)
                      (setq max 'many)
                     (throw 'many t))
                     (t
                      (unless optional
                        (setq min (1+ min)))
                      (setq max (1+ max)))))
         (help-function-arglist func)))
      (cons min max))))