In the primera division (first division), the attendance is usually in the ten-thousands. The Camp Nou, FC Barcelona's stadium, has a capacity of about 98,000. That many people have actually turned up in the Barcelona vs. Real Madrid rivalry games.
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I went to a soccer game in Atlanta last night, and there was over 50,000 people there. I would guess in other countries, where the sport is more popular, up to 100,000 people may show up.
It's impossible to get precise accounts for all the games on the planet, but it is, by far, the most popular spectator sport.