Someone else can feel free to correct me here if there's an official answer to this, but I think this probably falls under the category of "don't overthink it" ... The intent of the question is to see if you understand the concept of communication channels being any combination of two people among the project team, so take it at face value: if it says there are 20 people on the team, you should probably assume that you're one of the twenty unless the question states otherwise.
That last part is the key, though - make sure you read the question carefully. For example, if it says "20 other people" then you would be number 21. You should also pay attention to whether the question is really asking how many channels there are within the project, or how many you as the project manager are a part of (I've seen questions asked both ways, but can't remember which was a practice exam and which was the real thing). And of course, if you do the math for 20 and it isn't one of the choices, check to see if the answer for 21 is one of the options available. But in general, for a question like this, start with the simplest plain-language reading since a good exam question should be testing the concept, not your ability to parse ambiguous language.
(Incidentally, this advice isn't specific to the PMP exam; I actually took the PHR exam last week and marked several questions for review later because I was doing the exact same thing I just told you not to do and was splitting hairs with the language. When I came back to them during my review, however, I made a point to read the question again using the most straightforward interpretation, and the correct answer immediately became obvious when I did that.)
