WordPress Custom Template Pages by id

Example: page-xx.php or page-21.php

Page display -> http://codex.wordpress.org/Template_Hierarchy#Page_display

Template file used to render a static page (page post-type)

custom template file – The Page Template assigned to the Page. See get_page_templates().
page-{slug}.php – If the page slug is recent-news, WordPress will look to use page-recent-news.php
page-{id}.php – If the page ID is 6, WordPress will look to use page-6.php