How many U.S Forever stamps do you need to put on an SASE that is bound from the U.S to Canada? Thanks!

Wednesday is filming in Ireland as you are aware. I have US and Canadian stamps. What do you do if an address is UK, Ireland or foreign? Do you politely tell them that you don't have stamps? I would send them an envelope but couldn't do postage unless there is a trick where ours could be used. Are t...