All of them.
Right now I don't know which searches will turn up results from that magazine. I also don't see a way to ask Google which magazines it's scanned and archived up to this point. Looking here, I see Popular Science and Popular Mechanics, Jet, Ebony, Runner's World, Flea Market Fever (!) and others. It's safe to assume that Google will work on agreements with publishers to get whatever it can up there. Some magazines (think National Geographic or The New Yorker) simply make too much money selling collections of old issues to give them away for free. But we can hope that Google will prevail and get most of the good stuff over time.
In September, Google showed off that they have digitized copies of old newspapers going back to the 1920s. Again, it's frustrating that we can't get a list of the papers, but it currently includes at least the Pittsburgh Post-Gazette, the Saint Petersburg Times, and the Quebec Chronicle-Telegraph (apparently the oldest extant newspaper in North America). It's pretty amazing to go back and read news stories about Taft's election, all Googley-searchable, with the original ads right there, without having to go to the library and schlep through the microfiche.
Eaxmple of newspapery goodness here.
Ain't the internet neat?