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.
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, and each would come with a smart device for the app.