You can add location information to your Tweets, such as your city or precise location, from the web and via third-party applications. You always have the option to delete your Tweet location history. Learn more
It's from last year, but here's a historic rundown of why no one ever in the history of Washington DC has agreed on a crowd count https://www.nytimes.com/interactive/2017/01/18/us/politics/How-Crowds-at-the-Capitol-Have-Been-Counted.html …
One (very imperfect) measure is the number of rides on WMATA. https://www.washingtonpost.com/local/march-for-our-lives-huge-crowds-gather-for-rally-against-gun-violence-in-nations-capital/2018/03/24/4121b100-2f7d-11e8-b0b0-f706877db618_story.html …pic.twitter.com/6OJBIpB0PY
