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?
Cloud Platform Console Help has a page here: https://support.google.com/cloud/answer/6330231 which answers some of your queries.
Here's an extract:
"If you attempt to exceed your project limit, the console will prompt you to fill out a request form. This happens when you try to create a project but you have already reached your quota. The form will require you to specify the number of additional projects you need, along with their corresponding email accounts, billing accounts, and intended uses."
Best practices when requesting more projects are also listed.