A Project in the Certificate Manager is a set of insurance requirements, and all of the contractors you need to meet those requirements.
You can create as many projects as you'd like! Different managers use Project's differently. For example, let's say you require the same insurance for all of your contractors. You could create one Project with those insurance requirements, and add every single one of your contractors to that Project. Or, you could create multiple projects to organize and manage contractors separately. Feel free to create a unique project for each property, or event, or whatever makes the most sense to you.
The only time you absolutely must create multiple projects is to require different insurance from different types of contractors.