The number of sheets of paper you can send using one First Class Mail stamp in the United States varies by the weight or quality of the paper. The post office also counts the stamp and return address sticker in the total weight of the letter.
The weight of commonly-used 20-pound bond paper works out to about 5 pounds for a ream of 500 sheets, according to Howstuffworks.com. This works out to about 0.16 ounces per sheet. Allowing for a #10 standard envelope and stamp, four or possibly five pages will weigh in at around an ounce, according to BedTimes Magazine. Using a bigger envelope or heavier paper will decrease the number of sheets that can be sent.