A Raddit user created a map to illustrate that you can reach 29 countries within 500 miles (around 800 km) from the city of Vienna

— though we counted 30, plus Austria itself of course.
It seems a little hard to believe, so we dug into the comments and did some math. Here are some of the results.
Reddit users found this map super interesting and it sparked some debate as to the geographical boundaries of Vienna. Here is the list of the countries.
Denmark, Russia, Poland, Germany, Belarus, The Netherlands, Belgium, Luxembourg, Czechia, Slovakia, Ukraine, Moldova, Romania, Hungary, Switzerland, France, Italy, San Marino, Holy See, Slovenia, Croatia, Bosnia and Herzegovina, Serbia, Montenegro, Greece, Albania, Bulgaria, Kosovo, Liechtenstein, Monaco.
Further down the thread, a user has kindly listed 20 world capitals ordered by the number of countries within 500 miles, which is worth checking out.