You need a license for everyone involved. Each person who uses the shared contacts needs one. The owner of the contact list also needs one, even if they don’t use it. If the admin is the one paying for the subscription, they need a license too. So, if 5 people are using the contacts, plus the owner and the admin, that makes 7 licenses in total. However, if the owner of the list is also the one paying for the subscription, then only 6 licenses are needed.