Question: how many stamps to send a letter to luxembourg?
Answer: If you use "Forever" stamps, those are worth $0.47 each, so you'd need 2 of those and another 21¢ in postage, or the Post Office sells "International Forever" stamps for $1.15 (valid for one ounce). Assuming that it is a 1 ounce letter in a standard envelope (and not over 1/4" thick), then it is $1.15.