When we treat something to someone we often just show the place where guests can take food and suggest they take the food themselves as much as they want. In this case we say "Help yourself". Does this invitation change when we speak to several people, not to one person? Do we have to say "Help yourselves" in this case or it's always "Help yourself" even for many people?
(Not a Teacher)
Yes, it becomes "Help yourselves" when talking to more than one person.