How many applications can I create in one project from a single account and how many projects I can create in Firebase for Cloud Messaging?
Update from here:
There is indeed a quota on how many apps can be created in a project. The exact number depends on multiple factors, but it typically is in the lower 30s.
The quota cannot be changed, nor is it increased for paid projects.
How many applications can I create in one project from a single account?
I don't think there isn't any limit on how many apps you can link to a single project. AFAIK, I think it is actually encouraged for apps that are related when it comes to the Firebase services to be linked to a single project.
How many projects I can create in Firebase for Cloud Messaging?
I'm not sure what you mean for the for Cloud Messaging part, but I'm pretty sure that every project you create can be used for FCM. Anyways, I think it'd be safe to refer to the comment by @FrankvanPuffelen on this post:
There's no limit on the number of projects in general, but there is a limit on the number of projects on an unpaid plan. The exact limit for that varies, from what I understand.
PS: As per looking around the docs, I'm fairly sure that there is nothing explicitly mentioned/stated in the official documentation that can clearly answer your questions.