The estimate came from a reporter, I would take it with a grain of salt. The venue will probably be full by the time Trump speaks.....
Reporting on Varney (Cavuto just started), presser soon - FBI put out pictures of 2 guys that they think took a bomb out of a suitcase in NYC and walked away. Report (not sure if it was an assumption or was by the FBI) was that may be a terrorist cell.
That's why I looked it up - trust but verify.