Yesterday I reached 25, going by their status when I was in each country, by spending the day in Tallinn. I'd get two more, 27, going by current borders as Belarus is no longer part of the Soviet Russian empire, and Slovakia and Czechia split in the Velvet Divorce.
USA
Canada
Mexico
France
Belgium
UK (England and Wales)
Spain
former West Germany (now Germany)
Denmark
The Netherlands
Luxembourg
Switzerland
Austria
Italy
Vatican City
Sweden
former Yugoslavia (now Slovenia)
former Czecholovakia (now Czech Republic and Slovakia)
Poland
former USSR (now Belarus and Russia)
Finland
Liechtenstein
Japan
Israel
Estonia