Question: how many forever stamps do i need on a large envelope?
Answer: The postage required for a large envelope as of this answer (December 28, 2017) is $0.98 for the first ounce + $0.21 for each additional ounce. A Forever stamp is currently valued at $0.49 so you would need 2 Forever stamps for a 1 ounce legal-size envelope mailed within the USA.