Hi Pauline,

"Vous" can be either singular or plural. In this respect it is very much like the English "you". If you use "vous" as a polite form of the 2nd person singular you must also match anything that depends on it to be in singular as well. The same goes for "vous" in the 2nd person plural.

Vous vous êtes bien amusé? -- Did you enjoy yourself? (addressed at one person).
Vous vous êtes bien amusés? -- Did you enjoy yourselves? (addressed to a group).
Vous vous êtes bien amusées? -- Did you enjoy yourseves? (addressed to an all female group).

Note that even in English you would use "yourself" if "you" stood for a single person and "yourselves" in case of a group of persons. Not so much different than French, eh?

-- Chris (not a native speaker).

Sometimes it's a matter of context and if don't have all the information, you may not know.   For example "Je vous vois". You don't know if I see one or more than one person.  Without more information, you just don't know. And that's OK.  Always look for clues such as plural adjectives.