Corinthians has the biggest fanbase of the São Paulo based clubs and the 2nd biggest fanbase in the country.
With that being said, their successes have mostly happened in national competitions. They have a horrendous record in the Libertadores (2012 was an anomaly) and there’s not a big difference domestically to say that they’re better than the other ones (iirc Palmeiras even has more domestic titles than them).
Why the immensely large fanbase then? I’m not Brazilian, so how does it work when you come from a city like 4+ teams, how do you pick who you support and why are Corinthians #1? Is it like London where ppl often go with their neighbourhoods? Or simply whoever is winning the most at the moment? (The abundance of united fans in London)
