In some cases, you might want to have both a non-AMP and an AMP version of the same page, for example, a news article. Consider this: If Google Search finds the non-AMP version of that page, how does it know there’s an AMP version of it?
What if I only have one page?
If you only have one page, and that page is an AMP page, you must still add the canonical link to it, which will then simply point to itself: