[New York Times] Trump's Inauguration V Obama's: Comparing The Crowds
[New York Times] From Lincoln to Obama, How Crowds At The Capitol Have Been Counted
[The Atlantic] How Will We Know Trump's Inaugural Crowd Size?
[Live Science] How Is Crowd Size Estimated?
[Factcheck.org] The Facts on Crowd Size
The next one is an academic paper. Be warned! Head to page 27.
[Business Studies Journal] Estimating Crowd Size: A Multidisciplinary Review And Framework For Analysis
Finally, here is a gigapixel image of Trump's inauguration from CNN. The quality of this photo is so good that you'll likely find people you know in the crowd - if you knew people in the crowd.