First of all, you should know that each projects’ archive has its own style and structure. With other words, it means that a different number of projects listed there will impact the overall look-and-feel of your archive.
However, there’s a simple trick to change this particular number and display just as many projects as you want. To do so, please follow these steps: Go to Settings → Writing → Your Custom Content Types → Portfolio Projects, and change the value on this field.
Once you enter a different data, you will get the results you aimed for.