How to mention an amendment to the U.S. Constitution in text in APA style
Received an answer at the APA style blog: 

Regular number formatting applies when an amendment is mentioned as part of a sentence: spell out for the first through ninth amendments and use numerals thereafter
(e.g., “the fourth amendment,” “the 12th amendment”). 

http://blog.apastyle.org/apastyle/2010/06/how-to-cite-the-us-constitution-in-apa-style.html