Which of the following is the most important question to ask when analyzing your audience?

-what topic would they want me to write about?
-how many people are going to read my research paper?
-what do they already know about my topic?
-how would someone else write my research paper?

The most important question is 'What do they already know about my topic?' It would be rather repetitive if you talk about things that they already know about.

The amount of people reading your research paper is definitely not important. What's important is the information in your research paper.

Wondering how somebody else would write your paper isn't important; you should think about how you would write your paper to make it successful.