How many stamps you need to send a package depends on a number of different factors. The size of the letter or package, the weight of the package, where it is going (domestic or international), the speed it needs to get there and even the shape of the letter can all influence the price. Assuming you are sending a letter domestically than the major factor in the number of stamps needed is the weight. For anything one ounce or less 1 Forever stamp is needed. For each additional ounce up to 3.5 ounces an extra 15c stamp is required. Above 3.5 ounces you need to use postage flat rates which vary from ounce to ounce with the pricing going up to 13 ounces. Anything above that and you’ll need to start looking at package pricing options. If your letter is unbendable or not a rectangular shape it will be classed as “non-machinable” which incurs an extra 21c charge as it needs to be hand processed rather than automatically processed by a machine.