The number of sheets of paper that can be mailed with one stamp depends on the size and thickness of paper being used. For 20-pound paper, each 8 1/2-inch by 11-inch page weighs about 0.16 of an ounce, so five pages can be mailed with one stamp.
U.S. Postal Service mailing charges increase by the ounce, so a letter heavier than one ounce incurs extra postage over that covered by one first-class or Forever stamp. Letters that use 20-pound all-purpose paper can be mailed for one stamp if they contain up to five pages plus the envelope, which weighs about the same as one sheet of paper. Heavier papers, such as card stock or engraved stationery, need more postage for the same number of pages, which is easily figured by using the online USPS postage calculator.