Your NuCalm subscription comes with a one device license. However, if you'd like to share it with other members in your household, you could just as well put the app on an iPad/tablet or old phone so that the app could be used by anyone or taken with anyone at any time. You cannot have multiple sessions going at the same time.
If you are using NuCalm in a professional setting and want to be able to NuCalm multiple people at the same time, you would purchase multiple PRO systems.