Depends what you mean by "most." Clearly the biggest syndicator is Premiere, owned by Clear Channel, with Rush, Seacrest, Fox News, and lots more. This new combined company would be #2. Then you have a lot of other companies, like United Stations, Compass, Salem, Disney, Courtside, etc. There are other players in radio syndication, such as record labels, CDX, NASCAR, etc. I've heard Townsquare wants to do its own syndication. Several other larger radio groups would like to enter this field.
PLUS you have to understand that anti-trust doesn't happen just because a company gets bigger. The key thing the Justice Department looks for is if the bigger company uses its size to fix prices or stifle competition. That's more important than the amount of content controlled by one company. And keep in mind that the Justice Department approved the Sirius-XM merger, noting the size of the overall radio marketplace, and that satellite radio makes up less than 10% of it. Same with syndication. This new combined company would still be smaller than Clear Channel/Premiere. And syndication is still only 20% of the overall radio business.