It is mostly done by context. Let's say a guy in a seminar says
Haben Sie eine Mind Map gemalt?
If he is addressing the group it will be clear by his looking around and not talking to anyone in particular.
If he is addressing one person, then that will be clear by either his looking at the person or some conversation between the two that has been going on before.
So in face-to-face situations, non-verbal signs will do the job.
Sometimes it also context. When a politician says to someone from the opposite party in a talk show
Sie haben die Steuern erhöht.
then he probably means the plural as in "you guys in your party". Because it is unlikely that one single person is to blame for the tax hike.
In some situations the speaker might feel the urge to be crystal clear. Then, he can always say
to indicate the plural and some form of indication for an individual
Sie da/dort... (not very polite)
Sie, Frau Müller,...
Here are some examples in other cases
Ich verspreche Ihnen (allen), dass ich
Ich gebe Ihnen, Frau Schmidt, 10 Euro und Ihnen, Herr Müller, 20.
Ich habe Sie alle gesehen.
Ich habe Sie, Frau Schmidt, gesehen.