What is the size of the soccer fan base (the percentage or number of
the population that watches soccer) within each of the following 23
countries:

Argentina
Australia
Brazil 
Canada
Czech Republic
Finland 
Germany
Hong Kong
India
Ireland
Italy
Japan
Korea
Mexico 
Netherlands
Nigeria
Norway
Portugal
South Africa
Sweden
Turkey
UK
US

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I think that the only way to measure this is following the ticket
selling charts for every match on a entire tournament.
Got this information if needed.

But many fans don't get to the games, either because they are sold out
or because they live too far away from their favorite team and have to
watch TV.  If you limit the definition of fans to club members or
ticket sales or TV statistics ...