There is no exact standard size of a football field, but I think they are about 100m by 70m. A densely packed crowd is usually assumed to be about 4 square feet per person, so that makes about 23,000.
Of course, if you killed everybody in the world and cremated them or squashed them all together really tightly and piled them on top of each other, there would be no limit apart from the need to keep a few people alive to do the labour of piling up the bodies.