0 out of 9, but my housemate lived in the Marshall Islands for a year when he was about 2 years old, that's almost like visiting, right? (FWIW, the article fails to mention that Nauru's Our Airline flies BNE-INU-MAJ, which is both cheaper and faster than flying UA CNS-KWA.)
I suspect the first one I tick off that list will be Lichtenstein - when I finally manage to make it over to Europe, I intend to visit every one of its microstates...