I'm looking into where that number came from, but it appears to be the result of statistical sampling. In the meantime, the building was filled to capacity (note that, depending on the venue, just because a seat exists doesn't mean they fill it. 18,500 of 20,000 could still be considered "filled to capacity"), and there was a large overflow crowd of at least 5,000 according to the Dallas Police.
Note that Trump's projected overflow crowd is about the size of Beto's entire crowd for his rally on the same day. That wasn't unexpected, but it's an interesting comparison.