There are many countries that celebrate Pentecost. They may not celebrate Pentecost as heavily as they do Easter of Christmas, but they may celebrate it by decorating their church or simply acknowledging it. Any country that has Christians in it, then they probably could be considered a country that celebrates Pentecost. However, Pentecost is also celebrated by the Jewish faith.
Therefore, any country that has Jews in it would also celebrate Pentecost. All countries have some percentage of Christians, but only 20 countries list only 20 countries as having Christianity as the state religion. Most of the Jews live in either the United States or Israel. So, only 20 countries celebrate Pentecost, but there could be more since a lot of countries have Christians and Jews in them.