In a game of soccer 11 players constitute the official team but these players can be replaced with substitutes when the need arises. However this does not mean any tired player can just substitute with a new player. In a professional international match, the number of substitutions is limited to 3, but in other friendly soccer matches this number may be slightly increased. But such player substation should be validated by a credible reason such as injury, ineffectiveness or even a tactical switch. Sometimes some teams also use this as a timewasting technique when they are nearing the end of a game that is finely poised.
Each soccer match is officiated by a person who has full authority and whose decision is full and final. Referred to as a referee they need to be well versed in the Laws of the Game that govern the sport of soccer. But the referee does not face it alone, he has 2 assistant referees for help and advice. Sometimes in highly competitive environments like the world cup there is also the 4th official assisting the referee. A standard soccer match is played for 2 continuous sessions of 45 minutes each with a break between the two halves.