Let me take an example, I wanted to use Google Places API in my app. But due to usage limit it only allows me to use 1000/day requests for one key. So what I did was created multiple keys from different gmail accounts. I am using these multiple keys in one app. Is it allowed to use commercially?
Google already mentions that on the documents
You can use one Developers Console project to manage all of your work, or you can create multiple projects, depending on your development and collaboration needs. Consider whether you're collaborating with a different set of people, want to track usage differently, or need to set different traffic controls for different parts of your work. If so, breaking up your work into multiple projects might make sense. That said, you cannot use multiple projects to try to exceed API usage limits.
From :Google Docs : Creating and shutting down projects
So you cant use it multiple key for project its better to use the commercial package offer by google.