First of all is Real Madrid:

Real Madrid comes at the top of the list of football clubs with highest fans in the world. Success naturally attracts attention and creates affection! And that’s really the case with Real Madrid. They have won 9 Champion Leagues and 32 LaLiga titles so far which actually has turned them to the most successful club in Europe. Real have a tendency to buy superstars and that’s why they are called Galacticos which means “galactic” in Spanish, but in particular, refers to footballers at superstar levels.

The Second is FC Barcelona:

The second on the list of most popular football clubs in the world, Barcelona is said to have around 220 million supporters across the globe and is also the richest football club in the world with an annual revenue of more than €800 million. Maybe these days the name, Barcelona, is mostly associated with the name, Lionel Messi. Some believe the club has gained lots of supporters since the Argentinian superstar has shined for them, however, one cannot say the club’s own achievements have not played a part in its popularity if one knows that they have won 26 LaLiga titles and 30 Copa del Rey until now.

what's your opinion about the rest of the list?

1. Real Madrid
2. Barca
3. Manchester United
4. Chelsea
5. Juventus
6. Bayern Munich
7. PSG
8. Liverpool
9. Arsenal
10. Manchester City

Kinda suprised to see Chelsea above Juve and Bayern. Also dissapointed a bit to see PSG above Liverpool and Arsenal. Everyone other than Spurs in the English top 6 is there though which is nice. Big domination from the English and the Spanish club which is not really unexpected to see.

I wonder how they got their numbers. The finances are public, so that can be tracked, but how are the "fans" factored?

I would say the list currently is pretty accurate when I think of global football, but I know there are a ton of crazies in South America who breathe Boca, or River, or Corinthians, ect....

It is enough to make the finance public...
In my country, for example, if you ask a random guy what his favorite footballing nation is, he would say Manchester United. Not even kidding. This is true.

My point is that there is no way for a financial institutiion to know how many fans a football club has. Even if they manage to estimate a number, it is far more difficult how many of these fans are actually willing to par their hard earned money for their FC.
There will be a point where the banks will say things aren't appearing good with debts looming. Football clubs aren't countries to print money to finance their debts.

At the end of the day, the basic rule prevails. Cut down the operations cost, try to expand the fan base and have an eye on looming debts

PSG is fanboys only, outside of France. they only have fans in the Middle East because Qatar basically sponsored them 24/7 in their region, with firms such as Pepsi prop it up.