It's my goal to visit every country as well; my standard is usually one night spent there, if possible (its not likely I'll have a sleepover at the Vatican!).
There's ~194 'countries' recognized by the UN, so it's very possible to visit 5-10 per year and still make it. I'm currently at 86 countries, 72 are overnight visits.
It's harder to gauge 'coverage' when dealing with a bigger country. eg. I've been to Russia, but only Moscow; likewise only Rio in Brazil. But when in Liechtenstein I think we drove on every road in the country. So pretty good coverage there.
