arity

        # Method.arity

(from ruby core)
---
    meth.arity    -> integer

---

Returns an indication of the number of arguments accepted by a method.
Returns a nonnegative integer for methods that take a fixed number of
arguments. For Ruby methods that take a variable number of arguments,
returns -n-1, where n is the number of required arguments. Keyword
arguments will be considered as a single additional argument, that
argument being mandatory if any keyword argument is mandatory. For
methods written in C, returns -1 if the call takes a variable number of
arguments.

    class C
      def one;    end
      def two(a); end
      def three(*a);  end
      def four(a, b); end
      def five(a, b, *c);    end
      def six(a, b, *c, &d); end
      def seven(a, b, x:0); end
      def eight(x:, y:); end
      def nine(x:, y:, **z); end
      def ten(*a, x:, y:); end
    end
    c = C.new
    c.method(:one).arity     #=> 0
    c.method(:two).arity     #=> 1
    c.method(:three).arity   #=> -1
    c.method(:four).arity    #=> 2
    c.method(:five).arity    #=> -3
    c.method(:six).arity     #=> -3
    c.method(:seven).arity   #=> -3
    c.method(:eight).arity   #=> 1
    c.method(:nine).arity    #=> 1
    c.method(:ten).arity     #=> -2

    "cat".method(:size).arity      #=> 0
    "cat".method(:replace).arity   #=> 1
    "cat".method(:squeeze).arity   #=> -1
    "cat".method(:count).arity     #=> -1