In 1900 Europe consisted of 28 countries and more than 400 million inhabitants. The majority of them lived in the European part of Russia. The least inhabitants had the mountain state of Andorra.
On this website the administrative classification in 1900 is always stated for each country and for each district the associated municipalities, along with some information about population size. Not all countries have the same amount of information yet, but that will continue to increase in the coming period. Keep checking on Europa1900.eu and keep following on social media.