yeah i put 2 stamps on each of mine since i just started sending pictures, but for all of my other previous requests i put 1 stamp on my white envelope & 2 on my SASE
if you read the guidelines for the post office... you need to add an add'tl 13 cents if your envelope isn't "Machinable" (meaning, their machines can't electronically read them and put those orange barcode and so they have to do it by hand).. which i think is stupid... if it has a clasp/metal/other closures. add 13 cents... soo technically all the sase should be at 2 stamps...
but i'm very cheap... i would weigh my request to see how many stamps is actually needed. no need to waste a stamp!!