How many stamps are needed on a 10x13 inch clasp envelope if only one piece of paper is on it?

Usually one sheet of paper, even in a big envelope, is good enough for one stamp, since it is less than one ounce - as long as the paper isn't too heavy.
Updated on Wednesday, February 01 2012 at 01:50PM EST
Source: www.usps.com
Collections: one sheetone piecesheetstamp