No need to purchase separate subscriptions!
The only thing what matters is the subscription of the owner of the app.
If you have an Organization subscription, then make sure that the apps/projects you register on bitrise.io are registered for the Organization, or transfer them to the Organization.
If the owner of the app have an active subscription then the app/project will have the limits of the subscription, it doesn’t matter how many people are in the team of the app. You can add admins and developers to the app/organization, and they will be able to start builds, manage configuration (admins only), etc. - team members don’t have to have a subscription at all.
One note: the limits of the subscription are shared between all the apps/projects owned by that account. This means that if your Organization has two apps the concurrencies will be shared between the two apps. If the Organization has a 2 concurrencies subscription and App A starts two builds and then App B starts another build, then App B will have to wait until one of the builds of App A finishes.
Think about concurrencies as “build spots” or “build machines”. If you have a 2 concurrencies subscription then you can run 2 builds at the same time, regardless of how many apps you have. When the 3rd build is triggered it can’t start as you don’t have a free “build machine” or “build spot” which could work on the build, the 3rd build has to wait until one of the “build spots” finishes and can work on a new build.