Limbaugh audience size unclear. Estimates range from 30 million (Pat Buchanan on MSNBC), 20 million (Time magazine, ABC News), 19 million (Fox News), 14 million (CNN), or "14.2 million to about 25 million" (The Washington Post).
This is more impressive for NPR when you consider the number of outlets in a market that carry Limbaugh v. the number that carry NPR. OTOH, the fact that NPR is generally one affiliate per market makes it easier to track total network ratings.
This does not discredit Limbaugh's claim to be the king of talk, as NPR is generally thought of as news programming (although it has a number of individual talk shows).
If radio is really about chasing the winners and giving the public what they want, I would expect to see a flood of "All Things Considered" wannabees popping up.