Tuan,
Great to meet you. The number of free Cloud projects you can create (including Firebase projects) is based on your reputation with Google Cloud and varies, but it's small, as in ten or twenty, if you're new to the platform. If you upgrade projects, they no longer count against that limit (after an hour or two; needed to propagate the change).
Note also that deleted projects take some time to be completely wiped out (somewhere less than a month and more than a week) and still count against your free instances.
☼, Kato