For data as of 2005 there is a chart on the page on radio ownership and a table of the top 25 radio companies on the Project for Excellence in Journalism's report on the State of the News Media 2007. Click the table heading for "total" to sort by stations owned rather than news, and you will find that all of the top 25 own more than 25 stations. (Entercom's entry on the latter chart should read "103" not "3")

In addition to that and the above answer, you might want to check out a
similar question which was posted to the Radical Reference site in 2004. You can take a look at the previous list of resources for more sources of information on this subject.