Here at SmartEngage, we believe that a subscriber is a person. That person may have multiple methods in which you use to contact him/her, but it is still just a single subscriber.
This means that if you have an email address for a subscriber as well as their Facebook Messenger and Push Notification contact details - we will still only count this as 1 Single Subscriber.
We also do not double count subscribers across different Brand Avatar accounts. This means that if you have Dave as a subscriber on your Brand Avatar page for Yoga Fitness and Dave also appears as a subscriber on your Brand Avatar page for Anti-Aging skin cream - He is still only considered 1 subscriber for your account.