Swami Desikan has mentioned about 12 Thiruman kappu in his Panniru Namam; some adorn 13; is there a reason behind this? Should we adorn 13 Thiruman kappu after samashrayanam?

Swami has stated after applying 12 Thiruman kappu, if there is any leftover it has to be applied on the head which makes it 13 and there are no separate Thiruman kappu other than this.
One should not clean the remaining Thiruman or Srichurnam paste in the palm and it has to be applied on the head and there is a mantra to apply it on the head.