The Hillary Clinton campaign held a rally at Ohio State University on October 10, 2016. It was a post debate event and the day before voter registration ended in Ohio. News coverage stated it was a massive rally that drew 18,500 people. This figure was supplied by the U.S. Secret Service. There were videos and the photograph below on Twitter. By sifting through the Twitter accounts we found out that the rally was held in the South Oval of the OSU campus.
The image above shows a massive crowd that appears to occupy all of the South Oval. When we perform a CrowdSize study of this area as shown below and assume a very dense crowd, we come up with a crowd of about 26,700 in size. The 18,500 person number reported by the Secret Service seems plausible. The CrowdSize application lets us verify reports of large numbers of people at locations anywhere in the world and decide for ourselves the popularity of events.