Dear Matt:
This question only tells you that there are 20 stakeholders on the project. The project manager is also a project stakeholder. So if there are total 20 stakeholders on a project, the number of communication channels is 190.
If any question gives you the total number of stakeholders and ask to calculate the communication channels, just use that number to calculate the communication channels.
What Cornelius is referring to applies to scenarios similar to the following:
You are managing an XYZ project. You have 20 project team workers reporting to you. How many communication channels do you have in your project? In this case you will count yourself part of the team and the total communication channels will be 210.
Let me know if this resolves your confusion.
Warm regards,
Khurram