Notification Hub design for a single application

I have an Azure Notification hub in a namespace for sending notifications to a mobile app. If I go with a basic tier initially  and later if the active device count increases, is it better to upgrade to another higher plan or can we create another namespace for the same Mobile app.If we create 2 Namespaces for a single app, do we need to explicitly manage the sending of notification or the notification hub will do it automatically, as we have few users in 1 namespace and few users users in another namespace for the same Mobile app will it be a good approach?