Standard English has one word ("you") both for 2nd person singular and 2nd person plural. This makes stuff ambiguous. There's no ambiguity in Polish, 2nd person singular is "ty" and plural "wy".
So "masz" is the 'ty' version, while "macie", the 'wy' option. You will use the first, when talking to one person, and the other when talking to more than one person.