Answer:
As in more than one thank you: Thank You's. I believe that's how you do it because you put an apostrophe when there is more than one. Fun Fact: The word apostrophe is derived from a Greek word meaning "a turning away".
The plural of you is you. So "thank you" can be thanking a single person or a group. To the best of my knowledge it is only in slang Australian (Strine) that the word "yous" is used (no pun intended).