There is a quota on the number of projects you can create (+-20, more info here), but once you reach that quota, you can easily ask to increase this limit.
Adding more projects adds some overhead, but allows you to more easily do identity and access management and measuring costs.
You have to make a trade off based on the number of projects you will end up with and the number of people you will work with on these projects. Without having more information, I believe that having one project per app does make sense.
Having a seperate project for the backend and frontend seems less meaningful, this could also make the communication between backend and fronted services a bit more complicated.
I've been working on Google Cloud Platform from an year.
You can create @ the max. 3 projects but however you can the G-suite admin to check if you can increase the number of projects created.
FYI, I am talking about the initial FREE_TRAIL Google's authentication and policies are strict and 3+ projects would lead to blocking your account.
If you're creating a project for a Virtual Machine then I would suggest you provide more space ( like 400GB ) and then create VMs on top of it!!
Hope this helps!
One can ask for an increase in the projects quota limit, as indicated in the previous answer.
It is an excellent idea to have one project per app. In fact, this has been adopted as a rule at the end of 2016: each Cloud Platform project can contain only a single App Engine application and you cannot change the region after you create the App Engine application.
