Alan Gardner has posted an intriguing list of which newspaper strips are currently in 500 or more newspapers and which ones even crack the 1000 mark. He doesn't break out the ones in over 2000 but there are some. I believe Garfield, Peanuts and Blondie are all in over 2000 and there might be one or two more.
The list does not include any strips from the Tribune Media Syndicate. As I understand it, they did not respond to inquiries…but I also don't think they have any strips that are in 500 papers.
The number of papers a strip is in is significant but not necessarily the whole story. Some papers pay the minimum to run a strip and some pay a lot more. Syndicates have also been known to give strips free to very small papers just so they could inflate the size of the client list.
A prominent cartoonist once told me that if his strip was only in the daily and Sunday editions of the New York Post, the L.A. Times and the Chicago-Tribune, it would still be worth his time to produce it. Given declining newspaper revenues, I'm not sure that's still true but the point is that being in six newspapers could be more lucrative than being in two hundred if they were the right newspapers. (Daily and Sunday sales are counted separately…so if the Picayune Post-Dispatch runs your strip seven days a week, that's counted as two papers.) Many strips are also profitable because of merchandising or a large foreign sale. A strip like The Phantom, I know, is very popular overseas and its syndicate probably regards it as primarily an international offering with any U.S. sales being viewed as gravy.
Of note, of course, is that most of the strips that crack the 500 line are older ones and that many of them are being handled by folks other than the original creators. There are people who argue that a newspaper strip should end when its originator dies or retires. You can see why that doesn't happen and in most cases, I don't think the readers would like that. All the evidence suggests they're quite satisfied to see their old faves continue under new hands and wouldn't want to lose most of them for something new. I do understand the desire to see old strips go away…and of course, it's especially strong among cartoonists with new strips who covet that prime real estate. But it ain't gonna ever work like that.
It's also worth noting that few of the strips on this list usually employ any sort of day-to-day continuity or feature realistic characters. And it's very impressive that a self-syndicated strip — Chad Carpenter's Tundra — is more successful than many features that have big, powerful syndicates out there pushing them.
Not all that long ago, doing a newspaper strip was the ultimate dream of most cartoonists and there were an awful lot of non-cartoonists who saw it as a route to fame 'n' fortune. Every time there was a prominent article about how much money Charles Schulz made, the syndicates (there were a lot more of them then) would be deluged with samples. And as I've written here elsewhere, I'd get inundated with calls from friends of friends of friends who had, they were sure, the greatest idea ever for the "next Peanuts" and were looking for a cartoonist to draw up samples of it…for free. I don't get many of those calls these days. More often, it's e-mails from people who have a sure-to-succeed web comic who are looking for an artist who won't expect to be paid up front.