How do I set an attr_accessor for a dynamic instance variable?

I dynamically created an instance variable within my class:

class Mine
  attr_accessor :some_var

  def intialize
    @some_var = true
  end

  def my_number num
    self.instance_variable_set "@my_#{num}", num
  end
end

How do I make @my_#{num} now as an attr value?

e.g. I want to be able to do this:

dude = Mine.new
dude.my_number 1
dude.my_1
=> 1