Google sets a quota for how many projects can be created per billing account. The limit increases over the lifetime of your billing account based on a number of factors, but put simply: the longer your billing account exists, the more capacity it receives.
If you are nearing your project quota, you will see a warning message when you create a new project. If you have already reached your project capacity and try to create a new project, you will receive an error.
To request more project quota, fill out this form: