I has not test this idea yet, but if you have a event/function to RunOnServer, then the amount of your PlayerController BP is the amount of player(including listen server) connected.
If you want to keep tracking, then I think EvenPostLogin in GameMode(return a controller) and EventDestroyed in PlayerController can be your way to keep accounting player in and out without tick update with that function.
And, I like your silly soccer. 