How is a slot for each country decided on Pokemon World Championships?

Solution 1:

It's pretty simple: you need to qualify for it.

The tournaments have a minimum requirement of Championship Points (CP). Based on the region you live in you may need a different amount of points (making it "easier" to qualify in less populated regions). How many people from each country are invited depends on how many people meet the minimum requirement.

If you want to earn CP you need to compete in Championship Series events, such as regional championships or international championships or others. The bigger the event the more points you get.

If you win enough CP to meet the requirements you get invited. Simple, isn't it?

The CP needed to get invited can be found here.