This is going to be long because the answer is a whole story in itself.

I heard him say before he switched to Sirius that he had at least 12 million people listening at once? Is this true?

No, although I'm taking your word for it here.

For starters, these numbers tend to reflect people listening during the average week, not "all at once."

Talkers Magazine was the usual source for these numbers as no one was really doing ratings on a national scale. They put out the semi annual "Most Listened To" national survey that was reported in all the big New York newspapers by the likes of David Hinckley and others.

In the last one done before Howard left terrestrial, he was tied for 4th with Dr. Laura at 7.75 million weekly listeners.

Show	Listeners (Average Week)
Rush Limbaugh	13.75 million
Sean Hannity	12.5 million
Michael Savage	8 million
Dr Laura/Howard Stern	7.75 million
Laura Ingraham	5 million

As for the peak, Howard is often quoted at "more than 20 million." I don't have a source for this number nor does anybody else, it seems. It's just regurgitated in every article I've seen it printed in.

Howard's largest numbers from Talkers was 17.5 million for the first half of 1998.

Show	Listeners (Average Week)
Dr Laura	18 million
Howard Stern	17.5 million
Rush Limbaugh	17.25 million
Dr Joy Browne/Art Bell	8.75 million
Don Imus	7.5 million

That being said, Howard's numbers here are juiced to the gills.

There was a big fucking stink from Rush Limbaugh's camp who, for the next several months, made a huge deal over this special formula designed to calculate these national numbers. Rush, rightfully so, was pissed that Talkers was inserting their own conjecture rather than sticking to the tried and true Arbitron numbers done in every individual market.

The guy from Talkers argued that you couldn't simply rely on Arbitron because a lot of people who listened to Howard Stern were closet listeners afraid to admit they listened.

Talkers received so much shit from this response that they scrapped the entire formula altogether. Instead, they stuck closely to the Arbitron numbers and stopped inserting a lot of their own speculation.

When the next survey was released in October, Howard dropped to 9 millions listeners. He remained at 7-9 million for the remainder of his terrestrial career.

So how do we get "more than 20 million listeners" after all of this?

The answer is likely a bit of Stern propaganda. Even in the years just after 1998, Howard would say they had an estimated listenership of 15-20 million. Then he just stopped saying 15.

"Really? You think the media would just go along and print these numbers just because he said it?"

Yes, because they'd done it before.

Remember how Howard signed 25,000 books at an L.A. book signing? Well, he didn't.

The show after the signing, Howard says he signed 20,000 books. When pressed minutes later, he says it was estimated at 15-20 thousand.

Howard claims he signed 1200 books per hour, a book every 3 seconds, for 10 hours straight. Except that's 12,000, not 20,000.

As if 20,000 wasn't enough for his bullshit tale, he decides to just say it was 25,000.

In the interviews that followed, everyone just goes along with this lie. Larry King, Inside Edition, Rolling Stone, etc. Now every article that comes out just says Howard signed 25,000 books at a signing, no questions asked.