Letters can be mailed using one single U.S.A. First-Class 49 cent stamp per envelope, according to the United States Postal Service. It's the amount of postage that matters, not the number of stamps, so any number of stamps with a total value of 49 cents is acceptable.
The 49 cent rate applies to letters weighing up to one ounce. Another 21 cents is added for each additional ounce thereafter. Letters to international destinations that weigh less than an ounce require $1.15 postage.
A letter is a minimum of 3 1/2 inches by 5 inches and a maximum 6 1/8 inches by 11 1/2 inches. A letter also must be no more than 1/4 inch thick, with a maximum weight of 3.5 oz.