Pretty simple Use-case, clients buy our app and pay 20$/month.
We want to implement push notifications on both iOS and Android and are thinking of a scalable solution.
We thought of simply grouping all Apps inside one Firebase project, as we only need Push Notifications.
Google Firebase FAQ says:
A project is a container for apps across iOS, Android and web. While there is no restriction on number of apps within a project, adding an app can create one or more underlying OAuth 2.0 client IDs. There is a limit of around 30 client IDs that can be created within a single project.
You should ensure that all apps within a project are platform variants of the same application from an end user perspective. For example, if you develop a white label application, each independently labelled app should have its own Firebase project.
Would this limit us?
Do I need a client ID for each and every App inside my project?
firebaser here
What you're trying to do is known as whitelabeling, and is not an intended use-case of having multiple apps in a project. To not run into the limitation you've found, you will need to create a separate project for each client.
If you create multiple technical applications for a single client that are logically the same, those can exist in a single project. For example: the Android, iOS and Web versions of an app are typically using the same Firebase project, as may be specific app you build for the application administrators of your client. By having those apps be part of the same project, they can share backend resources.