I've noticed that often after a ball goes out a different ball is used. Also I don't know if the same ball will be used for both halves (plus extra time). So how many balls are used during a typical FIFA World Cup 2014 game?
FIFA's laws continue to state that
Also, I read here that Adidas manufactured and tested 20 balls for the Final World Cup game between Germany and Argentina.
A lot of people are against this system because home teams, specially in England use it to their advantage and ask the local ball boys to waste time during away throw-ins, etc.