If you click on PAYS (lower left corner of the game's window, next to the LP meter) it explains it. Like for Spain: 2 visits to Spain gets the first souvenir 10 visits gets the second souvenir 30 for the third 60 for the fourth 100 for the fifth and final souvenir So you have to keep going around the world to collect them.
Oh, just to add: London has the highest requirement at 150 visits. And you don't get anything for completing a set or collecting them all.