The European Union was started in the 1950s by France, Germany, Italy, Belgium, the Netherlands and Luxembourg. From 6 to 25. In the future the union might include Turky, Ukraine, Belarus, Croatia, Bosnia-Herzogovina, Montenegro, Serbia, Albania, Kosovo, Macedonia, Bulgaria, Romania, Moldova, Switzerland, Norway and Iceland. That would be all of Europe apart from Russia, which does not count. So, potentially, in the future the EU might contain 41 member-states. ...wow... ... And they say this is a big leap...
As of midnight the EU gains over 70 million people, rising its population to 455 million. Maybe 500 million if it were all 41. I do not know the populations of all there countries though.
In the very rare event that Russia joins (which it will not) the population would rise by a further 150 million (approximately) to about 650 million, but some of that would not even be geographically in Europe. But that would never happen anyway.