I’ve been lucky enough to visit 24 capital cities so far. They are:

• Valletta, Malta. Visited in February 2019.
• Andorra La Vella, Andorra. Visited in January 2019.

• Rome, Italy. Visited in October 2018
• San Marino, San Marino. Visited in October 2018.
• Vatican City, Vatican City. Visited in October 2018.
• Stockholm, Sweden. Visited in August 2018.
• Copenhagen, Denmark. Visited in May 2018.
• Luxembourg, Luxembourg. Visited in April 2018.
• Brussels, Belgium. Visited in April 2018.
• Madrid, Spain. Visited in January 2018.

• Amsterdam, Netherlands. Visited in October 2017.
• Budapest, Hungary. Visited in May 2017.
• Minsk, Belarus. Visited in February 2017.
• Vilnius, Lithuania. Visited in February 2017.
  
• Ljubljana, Slovenia. Visited in January 2017.

• Skopje, Macedonia. Visited in September 2016.
• Pristina, Kosovo. Visited in January 2016.
• Podgorica, Montenegro. Visited in January 2016.
• Belgrade, Serbia. Visited in January 2016.

• Paris, France. Visited in November 2015.

• Riga, Latvia. Visited in November 2014.
• Bucharest, Romania. Visited in July 2014.
• Lisbon, Portugal. Visited in May 2014.
• Kiev, Ukraine. Visited in March 2014.

• Warsaw, Poland. 
• London, United Kingdom
• Dublin, Ireland
• Berlin, Germany.

Not a capital, but still visited.