The title says it all... is there a quota of some sorts? I just deleted one agent and it still throws the error. I have a total of 6 (now 5) agents.
Yes, there is a quota on the number of projects you can get, and 5 sounds about right from memory. They will generally increase it upon request, but you need to do so through the cloud console.
Additionally, deleting a project won't immediately free up that space. The project will still count against your quota for about 30 days, during which it can be restored.