How many stamps do you need to send letters within the USA?
Depending on geographical location.
In the United States, you’ll need at least one first-class stamp for “Letters.” Non-first class mail is typically slower and less reliable. As of October 20, 2018, a single first-class price is $0.49. If you can’t find a stamp that sticks to paper well (due to water or other exterior elements), use two stamps affixed end to end with the opposite colors facing each other. This will cancel out the adhesive and give you a guaranteed stick for your postage. Preferably go with self licking stamps so that there’s no need for licking as a means of getting this done in person.