In Europe, there are a large number of dioceses principally centred in the countries of Italy, Spain, France, Ireland, and Poland.
Italy has the largest number of dioceses per capita of any country, although Brazil has more in total.
Andorra and San Marino are part of neighboring foreign dioceses, and so are covered by the Spanish and Italian conferences, respectively.
Dioceses are usually organized into ecclesiastical provinces headed by the archbishop seated in the designated metropolitan archdiocese.
Source:[1] This list is organized (when applicable) by episcopal conference and ecclesiastical province.