There's fewer than 10,000 professional Tennis players, and that's far more popular professionally than squash and other racquet sports so at a professional level you're talking more than 10 times football than tennis.
Good start... but everyone leaves out BADMINTON, with over 200 million players.
Badminton ALONE is almost as large as world football [!!!]
As I said above... who knew?
Thanks for commenting.
Team sports generally support larger numbers of professionals. In part because viewers are simply watching more people at the same time, but also because teams need redundancy.
If you're stepping away from organised activities, practically every kid in the world plays football, all you need is a ball, clearly that outnumbers sports that need equipment - Tennis and especially Squash.
By that metric Tag is probably the most popular sport.
Anyway you need a ball, several people, a goal, and a large open field to play something vaguely close to soccer. So it’s popularity varies wildly by area.