There is no way to answer this question, because most amateur players are not registered with any facility or team, so they would not be "counted", anywhere. Even if they were registered, not all clubs would reveal such numbers, in order to protect their members from potential privacy issues. The numbers of USTA and similar nationally recognized tennis associations around the world would tend to change from year to year, so that would not help, even if it were possible to track it down. One also cannot tell how by the number of tennis racquets and/or balls purchases, as many people own more than one racquet, buy more than one can of tennis balls, or share them with others, or continue to use old tennis balls.
There are many sixth grades that play football, but cannot be exactly counted due to the amount fo recreational, travel and school teams worldwide.
FOOTBALL (The one they actually play with feet ;)
Not me, sooo...
Football (Soccer) is the most popular and widely played sport worldwide. It is also the most commercialised giving it a multi-billion dollar lead over any other sport in the world.
Worldwide, there are literally billions more people who play and watch football than those who watch American football. The US is the only country where American football is more popular than football.
20,000,000 people play cricket worldwide today.