Interpret "twenty million Montag's running, soon, if the cameras caught him."

This is a rather frightening commentary of society. While on the run at the end of the book, Montag notices that every house is illuminated by the glow of the television. They are all intently watching the news unfold about Montag's run for freedom. If the helicopters spotted Montag, he would literally be on twenty million T.V. sets hence your quote. This isn't far from our own society where much of the news is given to us on real time.   

There are going to be 20 million people watching him